the bypass you have probably done is the actual N249 BYPASS.
That guide Wayne posted is more of a N249 removal.
remove the hose coming from the actuator to the N75 (remove it at N75 end) and clamp the hose off on the end. take it for a drive. that's one way to test the n75, but i couldn't imagine it due to the requested boost at 2200rpm will be nothing.
have you checked your MAF? unplug your MAF and go for a spin, if it runs better and smoother then that's your problem.
both of those things will cause a fault code to be logged, so you will need them both to be cleared after attempting them.