Ian,
Jim has standard cams and BD10s would make his lag situation worse.

Phil,
Jim thinks he has a modified exhaust housing, which will effect lag - so nothing to do with Karl .

FRS,
You can replicate the dyno to a certain degree by loading the car up in fifth. This puts the highest load on the turbo and will show you what boost it makes at what revs and is as close you can get to the loads the dyno can achieve.

Jim,
Harvey could do you a special turbo using the front of your T4 and the rear of a GT28, it makes the following (again lowered compression, polished head only (not ported) and one of Harvey's inlet cams):
Revs / torque / bhp / boost
3000 293.4 169.0 24
3500 341.7 230.6 27
4000 340.3 261.4 27
4500 341.6 294.9 27
5000 353.5 336.1 28
5500 354.2 371.8 28
6000 351.0 401.8 27
6500 337.5 418.4 27
7000 320.6 427.9 27
7500 280.5 402.7 27