You don't really need the IMR 18650, because a regular 18650 has plenty of output capability (current wise). IMR's are rated for extreme current outputs (I believe the 18650 IMR is rated for 20+ amps), but have less overall longevity.
That said, I use the IMR's that are recommended, in mine, because quite frankly I don't usually need any longer lasting battery, the IMR's are cheap, and they fit perfectly.
I can't imagine a situation where a provari would try to draw more power than a regular li-on 18650 would be able to output, because it has a built in cut-off preventing you from going to an extreme.