For most uses of an impact, the CFM isn't as critical since you aren't running the tool continuously. My single stage 30 gal, 3hp compressor handles my 1/2" impact just fine. The thing that improved the performance of my impact the most was a compressor with a higher output regulator letting me run the impact at 150psi. It'll knock off most things in one or two "ugga duggas", where it needed to bang on the bolts a lot longer at only 100psi.
For something like a die grinder, where you run it more continuously the CFM becomes more important, and I've got to stop and wait much more frequently for the tank to re-fill.