It doesn't matter what rule people make or what device you use to try to keep people within those rules, there will ALWAYS be someone that tries to cheat.

I'm not sure how you need to "calibrate" a GPS. I use them all the time in survey work and there's no calibration needed. It's a pretty simple device that spits out a location and a time. you then chart time elapsed, distance traveled, enter the car's weight, and you can calculate a horsepower number based on those three numbers. throw in transmission ratios if you want as well. If they're 100hp off, then it's because the operators aren't doing something right. Physics doesn't lie.

If you're getting a rise of 5' across the straight at sebring, then there's something else in the GPS unit- accelerometers or something as well.

As for the dyno fooling, that's a matter of policing and proper test methods. you can make any dyno read what you want it to read with tricks like heat soak and multiple computer programs and everything else. proper impound/testing would solve that.

But then again, for every rule you make, there's some unscrupulous bastage out there that tries to circumvent it. I see the same in SCCA too.