I have a VMPS larger and it sounds nice as a sub, but I can't comment on its max output, because I've never pushed it anywhere close to a point where it sounds stressed. Its certainly not a weenie, as you'd expect with such a large sub with so much Vd. The 18" woofers Wayne mentioned may be better, but at that point, why not just build the 7Pi-18? It'd probably be cheaper than a 4Pi + the 18" sub? Even if you don't have corners, you could still run the 18" in direct radiator mode.
-Ian