Ive just had a similar symptom, where my profec B holds 1 bar nicely from 3.5 to 5000, then it linearly drops off to 0.7bar at 7000, also on the rollers.
Mines got a 34mm inlet restrictor to the turbo, as its a rally car.
Is it worth investigating further? Its std ish except a front mount.
(I'm a bit new to tuning turbo'ed cars)