HP doesn't have anything to do with it & RPM will only have an impact if leak paths are opening up with the engine under load. Remember, the oil pump will be running in a relief mode most of the time when RPM is above ~3K unless you shrink the pump gears & stiffen the relief spring to achieve a linear pressure curve with RPM. The latter would mean you no longer have a high-volume pump.
Can you plumb a flow meter into the system & monitor it during dyno runs? Also, is it possible to include a sight glass or similar in the sump area?
All you're trying to find out is how much oil is pumped through the system in a given amount of time, and whether the system's return capabilities are able to keep up with the pump's inlet feed demands. This doesn't necessarily correlate with RPM or engine power output.