you can swap and GT shaft with any unit they all use the same bearing pack spec up to the 35r so yes they determining factor is how much material is in the turbine housing to allow it to happen thus you need to measure the outside and allow no less than 5mm per side as most garrett housings these days have low nickel content going less than this could mean trouble with high exhaust temps, main thing with any garrett gt or not is the dont like high response they surge like hell which makes for a narrow power band.
When it comes to turbine design garrett and borg warner need to catch up with the times the idea of removing blades and increasing the tip height along with high helix angles does not increase response study how a jet turbine is made youll get the idea, ive been lucky enough to have direct contact with some of the design guys at holset and if you compare apples with apples ive put some of the new HRC cores into GT turbine housings and picked up not just horse power but 5-800rpm better response and a great increase in torque.
The increase in response sort of kills the idea about BB turbos being better the other interesting thing is the power increase comes from running less boost, a evo i help out with was changed from a GT35 based unit to a HRC40rs i bring in it made 360kw on 35psi with the GT with the HRC it made 450kw on 28psi and came on 1500rpm sooner no change with fuel being used and its a plain bearing turbo but wheel designs totally different and smaller and ton more blades at both ends.
Garrett has its place in the market as do many others but they are falling behind the times billet wheels dont mean a thing yes a 10% increase in flow is great but not when you have to run more boost to get it, running more boost to get more power isnt the way to do it anymore.
The HRC35rs i supply on a stock SR20 does the same threshold as the stock GT25 and produces 310kw wheel on 18psi versus 230kw with over 400ft/lb torque cant be a bad thing and yes we have proven it over and over along with many other set ups we ever put a pair of them on a stock 2jz and made 1190hp on 25psi on pump, were also talking turbos half the price of a GT unit but for most people its purely a mind set.