My suspicion is that the turbo is not spinning freely in the bearings. Therefore the speed of the turbo is reduced at any given exhaust volume.

Remove the U-tube and attempt to spin the impeller with your fingers. It won't "spin freely" but it should rotate with just the slightest finger pressure..........almost wants to continue to spin when you remove your finger...........but won't quite do that.

If the capability to rotate is any less than the above..........you've got the bearings too tight for some reason.