the fm things completely different, certain wavelengths are reserved for military/aviation/police use etc, so the firmware will restrict you to legal frequencies in your area, extending that just removes the block, there is also a mod to increase power of the fm transmitter, again, due to legal issues.

afaik 3g signal is power variable, but this is controlled from the mast rather than the phone, if the mast thinks your signal is weak during a call it will instruct the handset to increase power output, boosting signal but hurting the battery. trying to override the power setting to max will drain your battery, and i believe the people who designed the 3g network foreseen this possibility and the mast will ignore your handset if it starts boosting its output without being instructed to. (i'm no expert and some/most/all of that may be wrong, this is just what i've picked up from others.)