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The ports are only able to be opened so much, and your also limited in the lift of the cam that will fit in a VG.
The reason modern 350 chevy engines make so much power compared to older ones is in the head port size and available cam lift is HUGE compared to older model 350 engines.
Engine make power based on the volume of air they can move, the peak power will be similar with the same twin turbo's used on a V6 or a much larger V10 engine with the turbos at full tilt.
This is because the turbo's air flow dictates the possible air flow that is the possible power.
How big the head ports are, combined with how much lift the cam has will dictate the response.
The ports and cams on a GTR engine are HUGE and is how the engine offers so much response.
A local shop uses our dyno to tune Ford GT's that have 5.4 liter V8's, they make about the same peak power as the Z with the same turbos, but the power band is much larger and the boost is lower at full tilt, because the engine has less resistance to flow.Here is a chart of a Z with twin GT3586R turbos, and a GT40 with the same twin GT3586R turbos. Blue is the Z and Red is Ford GT40 
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