Had tuners on gas and diesels, and while improvements of 100+HP on a turbo diesel was typical due to "SO much room to grow"
the SC 1865 gas tuner that's sitting in front of me gave about 20 hp and 30 lbft... certainly noticeable and as long as you didn't USE that increase all the time, like TT said, it won't pay for itself, but it will bounce the mpg up a bit...
anyone in the dfw area want it ?