Which is better on a track? i'd say T4 is better, it is not overstressed at this level of power and is less restrictive i.e. getting the gasses out of the engine, things tend to run abit cooler compared to the same engine with a T34 strapped on it. They may require some "driver input" to get the full potential out of them, but the T4 car would be better in the long run, it might not feel very exciting but then your not using the turbo to it's full potential at this level, where as the T34 will be very lively, but won't hold as much boost above 6.5k RPM, the T4 laughs in the face of this It's all swings and roundabouts, T34 will come on boost quicker but T4 will stay on boost longer and produce more of it, the trick is to learn how to drive the T4 powered car, it takes some getting used to.

I'd put the greens in the bin and get a set of greys on there, or 8 greens if it's on a proper RS500 setup. T4 can produce far more air than 4x 803 greens can deal with, boost control is also an important issue! It's all about the way the engine is setup, correctly specced cams + mapping can turn a T4 equipped lag monster into an absolute screamer

Your best off speaking to a specialist to get the most out of your setup i.e. TurboSystems, NMS, Reyland, MSD, MAD etc etc