There are more than two types of heat treatment can be done for that steel. The optimal treatment depends on wear mode. For example if you deal with abrasive wear (low cycle fatigue) the quenching temperature should be increased in order to obtain approx. 80% of austenite after quenching, temper is not needed at all. If the wear mode is less tough (high cycle fatigue) then quenching+tempering is needed. Since this steel is high-chromium it may be difficult to "normalize" it in classical understanding because it may undergo martensite transformation during air-cooling (it depends on the size of quenched parts). I think that in any case your parts will be quite brittle because of considerable amount of carbide phase. You may try to achieve high wear resistance for given wear conditions instead of high impact properties.

There will be microstructural differences after two treatments. After first treatment, pearlite with tempered martensite at the grain boundaries will be obtained while in second case only tempered martensite will be the final product. Presence of pearlite usually deteriorates the impact properties on the other hand,  steel with tempered martensite will have smaller impact transition temperature as compared to its pearlite counterpart. Due to smaller impact transition temperature, the impact properties of the steel obtained after second heat treatment will be on the better side.  

High carbon-high chromium wear resistant steels probably correspond to AISI D series tool steels which carbon and chromium contents are respectively between 1.5 and 2.5 % weight and above 12% weight. Due to the higher carbon content chromium carbides that formed at grain boundaries (eutectic phases) during the solidification sequence are present in such alloys. These carbides promote the higher wear resistance observed within these alloys. But in the same time, the chromium carbides are also reported to be very brittle regardless of the type of the applied stress (monotonic or cyclic stresses, see the two papers published at IJF (http://www.sciencedirect.com/science/article/pii/S014211230600199X) and recently at JMEP (http://link.springer.com/article/10.1007/s11665-015-1464-7)), and thus they can strongly impair the toughness resistance of high carbon-high chromium alloys. Impact toughness and wear resistance usually vary in opposite directions as alloys designed for higher impact toughness may not contain primary solidification carbides, unless they are subsequently dissolved or broken and fully distributed into finer particles inside the matrix from a thermomechanical process following the casting route. Actually, when enhanced impact toughness is seeked, the carbon content of steel alloys may remain sufficiently low (below 0.5%) together with a low content of carbides forming elements such as Cr, V, Mo, W, etc., in order to avoid primary carbides formation during casting. So I am wondering how you are expecting an enhanced impact toughness from a wear resistant steel alloy which is not designed for such an application because it contains brittle carbides. The link below gives general information and expected mechanical properties of various steels, especially regarding wear resistance and impact toughness. Tool steels designed for enhanced impact toughness belong to AISI A, H or S series (http://www.crucibleservice.com/eselector/general/generalpart1.html)

And even if you manage to obtain a matrix composed of a phase that is known to exhibit a high impact toughness such as bainite or spheroidized pearlite, the presence of primary brittle chromium carbides at grain boundaries should seriously affect the overall impact toughness properties.

You have quoted two different heat treatments that lead to the same hardness (30 HRC), and I guess that you are expecting improved impact toughness behavior from such a moderated hardness value. Concerning the QT heat treatment, you did not talk about the austenitizing temperature you performed but you mentioned that retained austenite can remain within the microstructure after a single tempering that followed quenching. In fact, higher austenitizing temperatures may lower the Martensitic start point (Ms) of your High Carbon-High Chromium alloy leading to the increase in the amount of retained austenite at “room” temperature. But in the same time, the higher austenitizing temperatures also yield a supersaturated austenite that can subsequently be transformed into martensite with higher hardness after tempering. If the purpose is to lower the hardness after tempering, then you should run tempering at a temperature close to Ac1 point to soften the matrix while forming ferrite + carbides from the previous martensite. Else you will have higher hardness values due to the presence of martensite associated with untransformed retained austenite if lower tempering temperatures are performed. But regardless of the tempering temperature, primary chromium carbides located at grain boundaries may remain within the microstructure and they will still impair the impact toughness properties of the alloy.

And if the second heat treatment quoted as normalizing is now considered, you can expect a matrix composed of pearlite and bainite in order to obtain a Hardness value of 30 HRC. But normalizing do not allow dissolution of brittle grain boundaries carbides and therefore, the impact toughness of your alloy should remain low.

And to come now to the role of retained austenite which remains from an incomplete tempering, one usually fears that such a phase transforms into martensite under unexpected increase of the temperature, or under either an externally applied stress or strain, leading to distortion (dimensional changes) or internal damage (cracks) within the part. Retained austenite has a low mechanical stability.

But to further the discussion and based on my personal experience with high carbon-high chromium alloy (see paper published at JMEP), I am quite surprised with the very low hardness value of 30 HRC you mentioned as example , because “real” pearlite is difficult to formed in such alloys, unless you achieved earlier a solidification process close to the equilibrium conditions (under very low cooling rates).

P.S: the personal post-print versions of the two papers are given in attachemnts.

http://www.crucibleservice.com/eselector/general/generalpart1.html

http://www.sciencedirect.com/science/article/pii/S014211230600199X

http://link.springer.com/article/10.1007/s11665-015-1464-7

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I believe one observe a higher resistance to brittle fracture/impact when retained austenite is present - especially when the operating/test temperature is low, e.g. less than 0 C (i.e. retained austenite has a FCC structure). In order to optimize the heat treatment of this alloy for impact properties, I would set up an experimental parametric study involving different hold temperatures in the austenite region, followed by various cooling rates from these temperatures, finally relatively low T tempering. Examine the impact properties and the characteristic microstructures obtained from these heat treatment studies. May be the optimum combination would be a low austenitization T, quench in RT  water, followed by a two stage tempering (ca. 400C + ca. 150C).