Mostly just changes the sound. I would say no on the 7 HP. Probably not any negative (or positive for that matter) side effects. I would also seriously doubt any change in MPG - people cite performance/MPG gains from a variety of mods but since driving habits are so varied, it's nigh impossible to prove any of them beyond dyno results (which wouldn't help you with MPG claims
AFAIK).
In terms of vehicle modifications voiding warranty, this has been much discussed here and across other forums. Essentially the dealer can only deny you warranty service if a modification or part you have installed caused (entirely or in part) the failure of a given component.