(Sorry for the delay in responding, but my wife delivered our first child last week! As a side-note, I requested sponsorship info from Antoine and I will more than likely become a sponsor of this forum.)
I bought a G-Tech competition for $350 a couple years ago, and I NEVER got reliable results. I spent an entire night on the road, recalibrating, doing 1/4 mile pulls, over and over again, and kept getting crazy results. That is actually what prompted me to implement the dyno and track performance tools in ProScan. The track performance tool in ProScan is dead on accurate as long as you don't get heavy wheelspin. Estimating hp and tq from acceleration data is much more difficult due to the # of variables involved, but in my opinion ProScan does a much better job than the G-Tech. ProScan can accurately read vehicle speed and RPMs, it doesn't "sense" RPMs through the cigarette lighter voltage. Also, ProScan doesn't depend on the accelerometers being perfectly level throughout the course of the run.
The only reliable way to measure hp and tq is on a chassis dyno. However, ProScan does a much better job than all other acceleration-based estimators, and with no calibration headaches... just plug up and go! Oh, and don't forget that ProScan is the cheapest OBD-II analysis tool available.
I look forward to sponsoring this forum and providing you guys with great service and support and great discounts.
Thanks!
David Gore