I think there's utility in a .22LR conversion for an AR versus building a dedicated .22AR. I think the 2A and CMMG units are both said to be pretty reliable.
Not sure if there's an accuracy issue running RF in a CF barrel, but on paper it sounds like a good idea.
I shoot my AR pistol with the cmmg 22 conversion more than any other. I've had a few crappy ammo issues but very reliable and fun. The accuracy beyond 25-50yds is not really that great but for up close and just training/plinking purposes it is awesome.