I can vouch as well.
The dyno was operated for 3 days with no 'adjustments' for anyone and similar atmospheric conditions throughout the weekend.
Generally, Jarrad drove the cars. It was a Mustang dyno.
The stock STi did 240hp and 227tq. (25% driveline loss of hp)
My ProTuned TD04 WRX (basically stage 2) did 229hp and and 235tq.
COBB's own Mustang dyno put down 225hp 223tq. I put down a bit more than COBB's car probably due to my ProTune and a few extra parts (P&P'd headers & TB, turbo inlet etc.) It was a very hot weekend or we all would have made a bit more power.
So...I'd say that the dyno was accurate, predictable, and properly run.
Whatever Vincent's dyno sheet shows...it's the truth. I know nothing about RSs but I CAN say that his dyno sheet is 100% real.