what were you expecting? the Turbo was pushing 85lb/min at about 60% compressor efficiency at that operating point and you made 720hp after drivetrain losses?
28psi isn't that much Emap though, I thought you'd be able to drive it harder even though it was pretending to be a heat gun by that point. what wastegate spring were you using and how were the pneumatics plumbed?
but really, if you were after much more power than that you picked the wrong turbo.
my test procedure for cars doing this is connect the boost line straight to the top port of the actuator, make sure my boost cut is set properly and go do gentle roll-ons down the highway (I mean on the dyno)