I've got one on my 2003 GT with auto tranny. The tranny definately shifts firmer at WOT, but I wasn't sure if I felt a difference in HP. The wife recently forgot I told her to use Premium, and put in 87 octane. I took it right home (one mile - slowly) and changed the program to the 87 octane setting.
Now I know it works because it definately feels like it has lost some HP. The best way to describe it is that with the Premium program there is a noticable increase in the cars pull starting around 2500-3000 RPM that is now gone. The pull is now pretty much steady and slower all through the RPM range.
I have to admit that I don't know if the Superchips 87 octane program is worse than the stock program, but I can't see why they would do that. End result is that as soon as I burn up this tank of gas I'm going back to Premium.