Ive had mine done - do a search for full details. Like it. Its a remap, so its just a "flash" rather than a hard-wired chip - makes it easy to undo. BHP gains for me were only 3bhp, too (from 164 at fly to 167), but torque has gone up from 147lb ft to 154lb ft.
These figures were achieved on a hot (91f) day in August on a rolling road. if such a high temp had a 2% reducing affect, bhp would be up from 166 to 171, and torque up from 150 to 157. If the reducing affect of the high temp was higher, then final figures would be higher, too.
Warranty? Yeah, you get 12months of cover from superchips, basically covering anything that goes wrong with the ECU itself.