Good question. I'd like to know as well since building an 8.5" barreled AR 9mm pistol. I've been wondering if subsonic would become supersonic due to the barrel length.
My understanding has been that the AK platform is difficult to suppress because of where the "gas tube" (or piston guide tube?) meets the gas block is vented.
Maybe I'm wrong, but it makes sense to me.
AR's, being so easy to build from scratch and customize into thousands of different variations, are the lego sets of the gun world, so really, we're all just collecting more lego's :p