GrimmSpeed is good quality. Are they talking about doing a custom protune, or just flashing a stage two map for you?
It will make it feel like a totally different car, much better power wise. The exhaust will be a little louder, but not drastically so. Anything that increases power will shorten the life of the engine/transmission. However, with a good stage two tune it shouldn't be that hard on the rest of the car.