Thanks Guys.
Ive borrowed a mate's book and it says 1 litre should suffice. Didn't think Demon Tweeks would do one, so that's nice to know. Get 10% off using their motorsport club. The Evo one looks sexy, but heavy. I'm trying to keep as much weight at the rear as poss. The sunken pump looks like trouble to change between stages, if it faults.
Good question has been raised. If the std pump runs out of flow on a one pump system, will I need to uprate both? Probably, unless the transient full power demand doesn't drain the mini tank.
I had fuel surge on my rally golf and it only lasted a couple of seconds.
Still thinking about it.