The unit I have fitted and have done about 1700 miles with is a GT1752 effectively. Changes from standard:
1) Increased the compressor from 49mm to 52mm
2) Larger shaft to take the extra load
3) both bearings uprated
The turbine is unchange by the upgrade and the unit look identical from the outside.
Results from the change should be about 2bar (29psi) available from 2500rpm right up to the red line. Mine makes 20psi at 2000rpm which is as near as damed standard boost at that point. So you won't see any difference to performance below 2000rpm. Once you hit that mark though, boost rockets up and you can cleanly burn far more fuel to get plenty of clean power.
The ECU will throw a fit if you go above about 2bar with the boost, which is good because it means you can not over boost this turbo. but you should always allow the car to warm up fully before pushing it hard and let it spin down after driving BEFORE turning the car off.
Options available are:
1) send a working turbo for full modification
2) send a working turbo for service and a replacement core (should be faster turn around but they are having difficulty aquiring cores at the moment)
3) buy a new reworked unit (obviously very expensive)
Option 1 will cost you about 600 notes but may take a week or more. So either the car is off the road or you find one on ebay to send in like I did (100 quid max you should get one for).
In addition, you will need to change the oil and filter, new manifold studs and nuts, new manifold gasket, new EGR gasket, new oil feed pipe and bolts (the bolt into the block has an internal oil filter so it is important to replace or clean this). All that lot will add another 100-200 quid, depending on where you go.
You should also spend maybe the first 50 miles not pushing the turbo hard, to let it bed in. It looks just the same and has the same pipe fittings, so if you can take the old one off, you can put the new one on.
If you are interested, give them a ring and ask to speak to Pete. Some of the other staff are not very helpfull unfortunately.
Still waiting to run it with all my water/methanol running and see how much power/torque it makes but I got 195hp/290lbft at the road wheel (est 229hp/360lbft at fly).
Graph of boost shown below, obviously, the boost tails off where they let off the accelerator.