Mine is currently on lock down at the LGS waiting for ATF paperwork:
However, my buddy (Karmapolice on here) has the factory DD upper on his KAC SBR lower:
He dicked around with the buffer weight, which caused some issues, but once he went back to the H buffer (what the DD factory gun comes with), there were no more issues. It's very light and quick. I have an SF buddy who was in the shop when I was filling out my paperwork and he says they loved theirs on last deployment.
Industry Disclosure: LGS employee
Semper Paratus Certified AR15 Armorer