Have wondered, too. There are some pretty well rated commanders at various levels (Balk, Model, Brandenberger, von Kluge, Kesselring, Kirchner, ...)
The general questions might be how different rating dimensions and rank affect choice of command level for a commander.
How do political, morale, initiative, administration play out at the different levels. What happens when a GenMaj or GenLt is rapidly promoted?
Have started a campaign vs AI using maximum APs to try out some ideas. So far have thought that the Inf/Mech ratings can be used to govern corps commander choices quite well, but seems a waste to put anyone of high merit to the AG or OKH level.
Thinking that Corps & Army assignments get the best effects out of the ratings. AG & OKH, not so much.