I think the sub 6 time is very plausible, I pulled 0-60 with a gtech of under 6 and confirmed it with a buddys radar gun from his police cruiser and a stop watch.
I actually felt the gtech was more accurate.
My main use of the gtech was to derive baseline numbers before I started modding the car. If I pull a base line of 5.9 then run a 5.1 even if the numbers are not exact it is still a .8sec improvement. I know there are variations but a gtech is better than a butt dyno.
Also, just to throw more fire on the flame. my wheel horsepower numbers on the gtech matched exactly what a dyno showed the same day.
In my opinion for someone doing performance mods a gtech is a great tool, it does not replace a dyno but its a great tool.