Heating up the oil and leaching a significant amount of oil flow if you're running two Borgs as twins.
As far as I'm aware the MPI kits were masterpower GT45s, which aren't really the same as a GTX like you're considering but I haven't been in the game long enough to see an original kit first hand.
As far as compressor flow matching a GT4202R almost perfectly matches a GT3788 which is closest to our stock AVNT Garrett. The issue lies in the fact that the GT42 turbine is small by diesel standards which reduces peak power potential and overall exhaust energy usage. You end up losing a bit off the top end and potentially losing some mileage at a cruise or while towing, and likely having to wastegate to balance response and top end flow. Overall it makes the Borg more attractive as an atmosphere over stock.
As was mentioned the GTX series in particular has a redesigned 11 blade milled compressor wheel which is easiest explained in layman's terms as reducing aerodynamic slip with the air at high shaft speeds. The reduced slip allows the wheel to keep effective laminar compressive flow higher into the boost range and increases peak flow at the limit of the chargers flow map, but it does nothing or even decreases performance where it would operate as an atmosphere.
Basically if you insisted on a Garrett you'd want a GT 45 or GT 47 turbine with a 4202 compressor with a custom billet wheel optimized for low pressure performance... Just get an S475 or run a GT4202 single where the smaller turbine allows a quicker spool and decent part throttle drive ability, and accept that you'll have to rev it up and run a higher stall converter to tow.
