air flow is determined by inlet manifoil pressure not by the size of the compressor that builds the pressure.

15psi boost is still 15 psi boost nomatter if your turbo is big or small or if you got a supercharger for that matter. 15 psi withing the plenum chamber will still be 15 psi.
is that so hard to understand?
the flow 15psi gives will still depend on plenum size and the flow capacity of the head.
at high rpm a bigger turbine-housing (or a turbine with cutback blades for that matter) will flow better than a small one.

if your turbo con't cope with the airflow needed at high rpm the boost will drop and you need a bigger compressor to maintain the boost. a smaller turbo (t-3) can't cope with a full stage 3 setup because it can't give enough air at high rpm to maintain the boost. thats the reason why a t-34 is part of a stage-3 setup.

running with a t-3 on stage 3 is fully possible, but the lifetime of the turbo will be drasticly reduced when you start pussing 20-25psi+ boost due to the extreme rpm the turbo will get.