My clutch can not cope with the torque peaking at 2300-2500rpm, so I don't see a need to push it further down the rev range and 6th gear is happy to accelerate from about 1300rpm, so again I don't see what benifit there would be from more boost earlier. The turbine flow is certainly an issue that can be addressed with trimming but then you lose some spool as I have seen on previous models or there is the external waste gate that I have shown to work well.
The issue with what you are suggesting is that the Vectra uses a bespoke manifold/turbine housing setup and as such a large amount of fabrication and cost would be involved in a swap to a GTB. The work I have done to my turbo requires just the comp wheel and housing machining, to give more torque than the clutch can handle and performance to shame the 2.8T V6 VXR 280PS, as it has done on Thunder Road at a lower output than now.
If a lightened turbine can allow me to maintain a simular spool threshold with a trim, that would be the most cost effective option for the masses and that is what I am looking for ultimately. With the uprated injectors I put in my mates Saab, the spool threshold is even lower than mine with the same compressor, so I will be moving to try that soon.
In money terms, a second hand turbo for mine is £100 and the work is around £450. I'd be pushing it to find a GTB2056 for anything like £100 and a manifold alone would set me back about £1000 because I have had several quotes. Then changing the oil lines on top and new exhaust coupling. I don't see many people wanting to shell £1500 + before they even get mapped or look at a clutch upgrade. This is the complete package as you refer to, pure performance is not the only consideration for me and if it was I would be installing a compound setup.