Danny,
I changed my wastegate on my GT3 and then had about a couple of months period where I couldn't get the car to boost right at all. Ended up taking it too 2 different places and they didn't seem capable of setting the car up correctly. One got it close and then I adjusted the wastegate to finish it off. It was still overboosting slightly at the top of the rev range and bizarrely when I fitted my dump valve, it sorted the problem out, that and disconnecting and reconnecting the battery to allow the car to learn its settings again. If you change wastegates, Dave Walters does a very nice alloy one which is great.
As for the way the GT3 drives generally, Mark TC pretty much has it summed up. Yours should definately starting boosting at 3000rpm and the car does feel a little bit slow for accelerating up to 3000rpm.
Also as Jonathon mentioned, the car should definately drive smoothly up to the 3000rpm point and then at that point you do get a bit of a stumble as the turbo switches in to start boosting. The GT3 is just one of those cars that you really have to 'drive' to get the best out of it.
Hope that you manage to get to the bottom of your problem soon as I remember mine when the turbo wasn't working correctly and it did make the car a displeasure to drive.
Regards
Chats