I'm wondering if as well as the whacking/torque element of an impact drill, the screwdriver bit used is important too.
I assume you are aware that there is a difference, between Phillips and Posidrive screws, drivers? The correct screwdriver, or bit, is essential to match the screw.
Bits intended for impact drivers, also tend to be tougher than normal. I have never failed to loosen tight screws, with a manual impact driver.