First I would look into a source for Helium and also what pressures it's storage vessels hold, and what regulator, or booster would be needed. I have seen claims of 100 FPS gains in some test guns.
Just one mans opinion, but I bought the Urban because I wanted a compact air rifle, a bit bigger than a PRod. As much pop as Travis is getting out of the PRod, I would bet one of his valves would work wonders on a Urban.