The 044 is a beast of a pump. It's also really loud. As I recall, the RS6 fuel pump(s) are modulated so that when not under load they aren't run at full voltage, which mitigates this somewhat. Not sure how that would work with an aftermarket pump though.

Did you check the genuineaudiparts.com for the OEM pump? Probably still expensive, but might be less than your dealer.

Regarding the pump pressure, the pressure varies with flow. I have data showing the 044 pump delivering well over 500 kPa, but that was into a 5 cylinder with smallish injectors.