Non-homogeneous austenite results in increased mechanical strength and stiffness in steels. As a negative effect, the reduction of the material porosity is a negative one. This is due to the accumulated defects in the crystalline structured material.

This is a very general question. The effect will depend on a variety of factors like e.g. on the austenitizing temperature and on the exact chemical composition. In principle, the hardenability is very sensitive to the local chemical composition and to the grain size of the austenite just before quenching. If your austenite is non-homogeneous, because the holding time was not long enough, the local content (specially of substitional elements) may vary from place to place, leading to inhomogeneuous properties. This is certainly more critical for alloyed steels. You may calculate how long the holding time should be, to attain an homogeneous austenite by solving the correponding diffusion equation. Or there are also useful "rules of thumb" for accessing this in industrial heat treatments. If your non-homogeneity comes from solidification, then you have a more complicated problem to get rid of.