Previous car, when i removed oem cat's in up-pipe and down-pipe, so up-pipe was straight pipe and down-pipe became 100cel sports cat after fitting the engine went into limp mode throttling engine & boost. Had to get re-map but the tuner was able to extract 25bhp and around 30 torque from just that mod, but entire rev range was much smoother and turbo spooled up so much quicker.
So expect you'll encounter the same with loss of back pressure, but keep us all informed on how it goes