I have been very happy with the "behind the firewall" location on both my RV-8 and Lousie's RV-6. Yes, when you flip on the electric boost pump, the fuel flow is biased high by 1.0 - 1.5 gph....which seems like a lot, but if you are only running it for a couple of minutes, it doesn't make much difference in the fuel totalizer values - and the "error" in fuel remaining is in a "good" direction (tells you that you have less than you really do).
That said, I recently mounted the red cube between the Fuel Servo and Spider on our RV-3 project, mostly because I simply could not find room behind the firewall on the tiny RV-3 without adding a considerable spaghetti-nest of fuel lines, more turns, etc. There was also essentially no good way to add it between the firewall and the mechanical fuel pump in the space available, so we mounted it in front of the oil sump, above the Servo - after consulting with the engine manufacturer and the makers of the Red Cube (EI). It's a neat installation, and I know of several airplanes that have quite a few hundred hours with the "cube" mounted to the sump bolts - but I don't think it would work as well with a cross-over exhaust (we have a four-pipe on the -3).
Walt's reference document is excellent BTW!
Paul