It also depends on the engine. No one engine will produce the same bhp, they'll all be slightly different. But they certainly understate the performance. I went from a mk 5 GTI (0-60 7.2 secs) straight into a mk 6 GTI (0-60 6.9 secs) and I can tell you there was a massive difference in performance, certainly much more than 0.3 secs.
As for the reason, it's been touched upon above - keeping it fast but not that fast avoids clashing with the R. It also keeps the GTI away from the raw performance of, say, the Megane Sport so that they can't be directly compared as they offer different things. The GTI has always been marketed as the all round hot hatch, doing everything well, giving it a headline 0-60 figure detracts from this.
I'm certainly hoping it goes sub 6 secs, every little helps! I knew it would, I've now got my figures crossed when mine turns up it'll be tuned to the max
