I have had them on gas and diesel trucks. The diesel truck it was awsome. It was a turbo diesel. More fuel and more turbo equals more power. It raises the HP up a few 100 and TQ a few 100!
Now, the gas motors only see maybe 25-45hp! Not much. Don't get me wrong, it bought one recently for a new truck I got "gas" motor. But, I mainly get them to remove the speed limiter.
I buy used, so for $100 it's worth the small HP and the limiter removed.
As the other stated. It's not of HP your going to see. It's worth it more for the speedo and firmer shifts if you have a auto. Then yes, get it.