Thought I'd save my findings online in case someone finds this relevant:
Using some of the methods described here:
Pics:
i.e.
Added 1 extra spring
Removed dart pegs
Removed plastic shroud
Re-lubricated using silicon grease
Added a bit of epoxy for extra reinforcement
Spring:
Used some 2 random springs, I'm unsure of their specification, unfortunately - they appear to have similar size/stiffness to stock, which is in turn similar to stock Australian elite springs. It likely makes little difference though, since the plunger tubes are small/resistance is low/there's some air leakage between the cylinder/plunger.
Rotation mech:
Even after removing the shroud, as you can see in the video, there seems to be some rotation issues with FVJs. They don't actually occur that often during operation, since the pump typically moves slightly faster, but it's worth noting.
Performance test:
Performance data (fps):
62.1, 61.6, 59.8, 56.4, 59.7, 69.5, 67.3
High of 69.5, average of 62.4, SD 4.60 with a sample size of 7 shots. Just under stock American elite performance.
Also I'll try to take better pictures next time I make notes, only just realising the pictures aren't that comprehensive.