You're definately right, it is hard, but it can be done, and it has been done. I don't understand the last statement though - If the dyno is done right (with it SAE corrected to even factor out such things as a change in elevation, temperature and humidity), I can't think of a more accurate way to measure output other than an engine dyno. A guess is just that, only a guess.
Anyway, after 4 days here's what I've found:
1) Race Shop Joe has a stock displacement 12v makeing 205whp/191wtq. He ran 12.49 at 108.8mph.
http://www.race-shop.net/
2) BillyVR6 on Vortex
vwisthebest (12:40 PM 9-27-2005): how much power did your NA 2.8 12v make?
billyVR6 (4:38 PM 9-27-2005): 194whp with the schrick 268's... went 12.87
unknown whp with the techtonics 288's (never dynoed)... went 12.83
3) 95GLX on the Vortex had a NA 2.8L 12v VR6 making 223.3whp/197.2wtq & 222.8whp/201.5wtq
http://forums.vwvortex.com/zerothread?id=1354649