I knew a guy who had a GTi. It might have been an 07, it was a DSG. Anyways, he dynoed at the wheels almost exactly what the car was rated for at the crank, I think he put down something like 200/200 totally stock. Then with a CAI/downpipe/tune it put down like 280/330 or something. It was surprisingly fast actually.
I think it ran a 13.4 @ 102 or something like that. He upgraded the turbo, fuel system and installed an LSD and ended up getting into the mid 12's.
So, with just a cai/downpipe/tune that mk5 should pull you, especially on top end. With a larger turbo it shouldn't even be a race.
99 Chevrolet Camaro Z28 M6 - 6.676 @ 103 in 1/8, 10.512 @ 130.2 on street tires, H/C/I
93 Eagle Talon TSI AWD 5spd - Built 6 Bolt, 16G Evo3, HKS 272 Cams - under construction
94 Mustang GT - 5.0 bored and stroked to 347, Vortech V1 @ 10 PSI, Tremec TKO 600 - PIA