Im going to do a comparison soon with the high flow K&N kit, the standard Rhino UMP filter, and a larger UMP filter based on the RZR/Teryx Can. Ill be sure to post my findings. (It will be on a Benchmark 686, so flowing alot of air over stock).
FYI, I did 60 mph on the dash with a full desert race cage and 26" bighorns with the UMP filter, HSR45, ProCircuit Duals, CDI and machined sheave (stock motor). So I think it flows pretty good.
I was only able to do 54 on the same stretch with the stock carb/intake/exhaust, same CDI and mach. sheave. 25" bighorns.
Plus the UMP filters out way more dirt, and lasts longer than any other filter between cleanings. Those two things make it the winner in my book, especially for dusty conditions, even with some power loss from the restriction. Sand on the other hand, where max HP is important, and not much dust, I would probably do the AFE.