The GP2X defaults to running at 200mh to save battery life, because most things run perfectly at that speed.
All units can be set to run at higher speeds on certain programs if they need it (Amiga emulator, SNES, Quake, MAME, Neo Geo mainly).
Usually each unit's maximum speed is random, anywhere from 240mhz to 295mhz (on Mk2's it's rare to go higher), but the "270mhz powerpack" has been tested and guaranteed to run at at least 270mhz.
If money's no object, you may as well go for that as it will help with at least Amiga and SNES (nothing else really needs above 250mhz).