Well, that would of course be possible, but then you could also change the CPU frequency for the other 2 modules (Sargon 2.5 and Sandy), but they go crazy with these higer speed. So that's not a good idea.
I don't think this is needed, because I've already tried 20MHz for the CPU on the Steinitz module (with the VIA6522 frequency unchanged at 2MHz), and everything worked fine - just the clock is running a little bit faster (but only a few %).