No amount of overclocking would allow PS2 chipset to compress video at the same time as playing games and movies. Also, you would stand a good chance to run into compatibility problems if you tried running PS2 software on overclocked chipset.
Speaking of which, EE was already overclockable over 3years ago when it was still on .25micron (GS was more problematic, but that is just consistent with the lower yields(relative to EE) it had in begining).