Is that enough to account for the difference?
The biggest difference is the turbo hotside/turbine being bigger.
That affects VE in a massive way at higher boost levels. At lower boost the difference is very small, but as you go higher it starts to become larger and larger, as the OEM hotside/turbine wheel would be creating massive backpressure at those levels, that just isn't there with the upgraded turbo.
But as I wrote before, if there's a small boost leak it'll show up the same way.