Blown Alky GPM dropping at launch

I have recently switched to an 800 Waterman and have done 6 runs and have noticed the GPM dropping from about 0.4 and not recovered until 1.4s into the run.
It's a bracket car, Engine basics are 511KB, BAE 6 heads, 1471 hi helix 36% over 35psi at 8000 and I run a glide style clutch so leave at idle (2500rpm)
Previously ran a smaller (unknown brand) vane pump which was about 14.5gpm free flow but it did not have this issue at all.
Everything else is the same on the car, just adjusted the main pill, then also added a pump loop to bring the main back into a better range. There are no other lean-outs or anything on the system.
The drop occurred with and without the pump loop.

First thought was fuel slosh / sucking air in the tank, but the previous pump didn't have this issue.

The waterman pump wasn't recently flowed but is making 16.2gpm @ 141psi @ 8000rpm in high gear.
Could something be happening inside the pump due to leaving off idle?

The clutch settings are still way loose, so was focusing on getting that closer. The previous fingers were damaged and the new ones are a slightly different height to the pivot, so it's taking a lot more weight to engage. Could the clutch grabbing late and pulling the engine down be causing this issue with the pump?

Yellow is GPM to engine, Blue is return GPM.
Red, Orange and Green are Engine, clutch and driveshaft.

Cheers