I have several points here. I am aware that the ideas I propose assume an ideal research context in which the researcher is not limited by time or money constraints. However, you might find one or two of them useful.
1) Which other statisitical measures were you thinking of using together with readability?
2) Do you intend to measure readability entirely through statistical means? In my view, it would be far better to mix quantitative readability measures, such as Flesch Kincaid, Gunning Fog or SMOG with intersubjective readability ratings. The data triangulation involved in such an approach can both make the research more robust and enable you to critically evaluate which aspects of your research can be improved in future studies: on the one hand, the areas in which the quantitative and qualitative measures coincide can be considered to provide more reliable data than the areas where there is a divergence; on the other hand, by searching for reasons for divergent data, you may uncover flaws or limitations in aspects of your research method.
3) Are you thinking of measuring translation loss in a specific genre and, if so, which one? If you do have a specific genre in mind, you can use two complementary means of identifying the linguistic features of the genre that are most likely to lead to translation loss: you can look through any previous research to find out which linguistic features were found to be problematic; you can examine an existing parallel corpus of genre specific texts to see which features regularly cause problems. If no such corpus exists you could create one. ( I did warn you that I would be assuming an ideal research context!).
You are absolutely right: there is no relationship between genre-specific features and statistical accuracy. However, if you are going to analyze a specific type of texts, then it could be useful to focus on the specific issues that might affect readability in those texts, in addition to the general criteria that you would be evaluating with your readability tool(s).