Added H.P.? Nope
Added MPG? Not sure, but I doubt it.
Well worth the money? YEP!

The sprint booster basically gives the car better throttle response. If you own a crossfire or any other 'drive-by-wire' (electronic throttle control) vehicle, you are probably familiar with the lag that this system can cause. The crossfire is probably the worst that I've driven, regarding this lag (both SRT and Non SRT). Now, the car is VERY responsive, VERY 'punchy', and seems MUCH more eager to get off the line and get moving. If it doesn't help acceleration, the only reason is because it has a tendency to break the tires loose now. It definitely FEELS like it helps acceleration.

This device basically takes away this lag by making your pedal input 'quicker'.

In stock form, you have to press the throttle down 80% of the travel to achieve 100% throttle opening.

I believe it only takes 40% of the actual motion to open the throttle 100% now (I could be wrong on this).

So, you basically open the throttle twice as fast - resulting in what feels like less lag, more horsepower, quicker acceleration, etc.

Some people argue that they can smash the pedal fast enough to make the sprint booster unnecessary. I have raced everything from 125cc shifter Karts to 10 sec 1/4 mile cars and F1 cars, and know how to push the skinny pedal plenty fast. The sprint booster still put a smile on my face!

Take it for what it's worth, but don't knock it until you try it.

The biggest down side would be in wet/ice/snow conditions where you will be more likely to break the rearend loose. I am in Texas and the only ice we usually see comes from our Freezer. As far as rain, it stays in the garage on those days. It is manageable anyway, but definitely easier to break it loose.
You may also be more prone to drive a bit more spirited and use a little more fuel.

SQ