Turbos have changed a lot over the years. But most people out there in modification land stay away from the best technology and stick with old stuff, which while good in it's day, is some way behind best practice. Take the WRX. When it ran the 2 litre engine, it came with a TD04 while the STI came with a VF35. Earlier STI's came with different turbos, such as the VF28. But the STI's (with VF35) were renowned for their turbo lag,while the TD04's just ran out of puff. But they spooled up a lot earlier and they were used for a very long time.
My 1999 WRX also came with a TD04. I initially changed it for a VF24- very similar to a VF28 that the earlier STI's came with. Boost came on 200 RPM later, but then they would also run out of puff in the top end, albeit better than the TD04. At that time and for a long period of time afterwards, the only way you could really have the low down torque of a small turbo with something that went hard at the top was with a twin scroll. But you had to change your exhaust manifolds, sump, oil pick up and so on and the car sounded terrible with them.
So I elected to wait until technology came up with something better. And it did. Billet wheel turbos give me excellent low down torque- stunning actually, and then go on to produce more power than not just the VF35, but the bigger VF34 as well. I know a bloke with a VF34 with same mods as my car, and same tuner as well. At 3,000RPM, my car has twice the power his does. It then goes on to produce a slightly higher maximum power, but it holds the power over a much wider rev range.
With a diesel, all you want is low down grunt, but whether there is a billet wheel turbo for diesels I have no idea. But it may be an avenue you wish to explore.
