Where am I getting my information from?!?! Knowledge of the cars and ownership.... not from the back of a magazine!
For a start an 02 WRX scoob doesn't have the 2.5L engine in it, it is a 2L flat 4 as per the classics..... the only way that has the 2.5L in it is if it has been converted!!!
The gearboxes ARE different, the WRX has a 5 speed box and the STI has a 6 speed box it also has different Diffs as well, the STI has DCCD so if he did have an STI he could manually shove most of the power to the arse end.
The poster mentioned that the WRX had been modded, I can only assume that he has had the clutch replaced with something decent as the normal WRX clutches don't last long under high power and high Torque
I've had a classic WRX running 310bhp, And Evo 7 GSR running 350bhp and a P1 running 333bhp (after EcuTek) and the P1 was quicker than all of them thanks to the magic of prodrive (again gearbox change from a std STI), it's not down to power alone, it's got alot to do with gearing!
For example..... A massivley powered WRX would get battered down a drag strip by a significantly less powered STI RA due to it's gearing, fair play the RA tops out at about 120mph ish but the speed in which it gets there means the WRX has no chance at all and top end is insignificant in a drag race!
You are also assuming that the Supra is a UK spec TT, it probably isn't a UK car making it a 280bhp car.....
All in all you've got cars that will have very similar figures but the impreza will have traction and weight on his side and my money would be on the impreza, unless you are going to run miles in which case the supra probably would reel it in and then pass!