Actually from those i know old gpscumm has used 166 mhz. Also in gpengine you can set clock to values higher than 133, so better dont do it. 133 mhz is enough to play gpengine. And i am not sure if than Media Player doesn't use higher clock values than 133mhz
The current ver of media player is certainly not clocked to 166, coz its stable on my GP32, and the previous version wasn't. With this one, I've been experimenting LOTS and not having problems, so its at most ~140, so at a guess 133 (since 7 MHz ain't going to make a lot of difference).
Hopefully, this will force devvers to optimise their programs a bit more. Not saying that theyre not doing a fantastic job, but as we all know, when you limit something, things only get better. Good example of this is Doom level design. Its amazing how much they can cram into such a simple 3d engine.