The big advantages of a GCG ball bearing high flow;
1. Fits back on exactly as it came off, you don't have to make parts, buy parts or pay for parts. All the necessary gaskets and seals come with the turbo. Pay $1750 and that's it, fit it and tune for it.
2. The result is well known, stick some cams with the high flow, decent FMIC, injectors, fuel pump and 3" exhaust and you will get 265 rwkw if it is tuned correctly
3. More power than standard everywhere, that's from idle to redline. You have to be very good at choosing turbos to duplicate that.
4. Legal, not defects for fitting a high flow, looks exactly the same as a standard turbo.
5. It's a ball bearing water cooled core (not a plain bearing, oil cooled core).
The advantages of an aftermarket (replacement turbo);
1. Any power output you like, trade off response and lag to max power to whatever level you want
2. All price ranges, plain bearing oil cooled turbos are cheaper than high flows
3. Cheap, second hand turbos are always available
4. High mount, external wastegate whatever you want/can afford
5. No guaranteed result, if you choose right you will get a good result, if you choose wrong you won't
As they say horses for courses, if you are after a simple bolt, legal, no more to do solution and your power target is up to 265 rwkw then you can't beat a high flow.
cheers