Lowering the frequencies does affect the temperature. Additionally, consider this: lower frequencies might allow for further voltage reduction, leading to even greater temperature decreases !
This still needs to be tested on a FAT model... something I unfortunately don't have.
Otherwise, I would try it instantly, at least for science and fun haha !
The XMB isn't even sluggish for me. If you have the waves in the background, the framerate might drop to 30 or even 20 FPS. However, with a static background, I'm still at 60 FPS, and navigating the XMB remains smooth.
50MHz core seems risky to me, though (at least without a flasher) :')