The standard turbo runs a T3 style compressor wheel with a T25 style turbine wheel with T3 housings. The S2 R33s have a Nylon compressor wheel, the S1 have a steel - both have a ceramic turbine wheel and a ball bearing core.
HKS don't even make the GT2540R any more do they?? Anyway, the GTR version of the GT2540R won't bolt directly on as they are a T2x flange BUT they did do a T3 bolton version specifically for R33 GTS25ts. As far as I know the GT2540R bolton turbo has been effectively superceded by the HKS GT-RS kit, which you can get as a bolt on for the R33.
These things come sprung to about 13psi, so you will need to upgrade the ECU, fuel pump and realistically injectors to have it run safely or get the best out of it. Depending on the fuel and your tuner, 14psi is probably not very sensible to go past on a unmodified RB25DET - though you can make some serious power with them.
On my R33, I am running a .63a/r Garrett GT3076R turbo which bolted onto the standard manifold (albeit with a 10mm spacer) and mounted a TiAL 38mm external wastegate onto it. This needed a custom dump pipe, intake, oil and water lines made up and I now run an aftermarket ECU that uses a MAP sensor, have 555cc injectors and a big fuel pump as well as a decent intercooler to support what the turbo wants to do.
The thing is making in the territory of 370whp at the moment with an untouched motor and makes full boost around 3600rpm.
OK, maybe I should re-iterate. You SHOULDN'T run a GT3076R (or any bigger turbo) with stock ECU on an RB25. Its just asking for trouble.That's true, and i understand that. But by replacing stock turbo with a gt3076r, in boost therms what will be max boost ( air flow ) you can run with stock injectors and stock ecu? ( how much bigger is a gt3076r than a t28? )