Can't speak for the mk5, but I had a diff fitted to my old mkiv gti. This car was an early model, AGU engine, and I had fitted a K04-23 turbo and all supporting parts, so was effectively an S3/TT engine, but mapped to 250+ (no idea on exact figure as I never had it on a dyno before it was killed by a 1-series).
From my experience - the Quaife diff absolutely transformed the car. As it originally had no TC and was throttle cable, whilst it was nicely controllable under foot, the engine could still spin up the wheels in the wet or dry when deliberately provoking it.
On my first drive after the diff was fitted, the traction was transformed - it was after a rain shower. The car just dug in and got on, whereas before it would have scrabbled, understeered, and spun the wheels until letting off the throttle, or traction was gained.
All this was at low speed, out of junctions and going round mini roundabouts. I never had the chance to go on track to test high speed cornering changes before the car was killed, but I do disagree with the comment that the Quaife is ineffective at low speed, as in my experience, it simply isn't - it is amazing. I had to learn a different way that the car drove...
In ten years of ownership/modding my mkiv, it truly was one of the mods that I said to myself 'why didn't I do this earlier'.
