i've gone from t34 0.63 exhaust to gt25/40r 0.64 and there's big difference in spool up time, this is on 2.1 zvh.
My t34 started spooling up at 3200rpm where as the gt series came on boost at 2500rpm, also i ran 24psi on the t34 on greens and i still had loads of fuel left in my map but with the gt turbo injectors maxed out at 19psi power differential i gained 40bhp over the t34 but gt turbo runs with tubular manifold. t34 small compared to the gt25/40r