Ok, did some more testing and checked the fill and exit valves.
The fill valve is directly threaded into the back face of the magazine and there is no syphon tube so it is easy to completely fill the gas chamber with liquid propane (or near enough!) Meaning there is little gas available for firing a BB, and shots in quick succession lead to a drop in power due to it taking longer to replenish the gas pressure by boiling off the liquid propane.
8-10sec fill seems optimal.
The exit valves on one of the mags was a little sticky so I removed it, lubed with very light silicone oil, wiped the excess off, and refitted.
Much more consistent FPS and even gained 5FPS due to efficiency. I did the other mag and found it didn’t make much difference, but that one was always working better any way.
Hope this helps anyone else who gets the G&P MWS mags and finds some less than great performance.