The RL is not a luxury sedan of the same caliber of the LS, S Class, 7 series, etc whether its FWD, AWD, or RWD. The A8 however is an entirely different caliber of vehicle. I don't think you'd find many enthusiasts who wouldn't put it in the same class as the LS.
I define cars by how they are built and the performance they deliver, its a shame that you just automatically discount something because of the setup of its drivetrain. The A8 is a really, really nice car. Go test drive one. Great interior, great quality, reasonably priced. To say its not a "true luxury car" because its AWD is just...not accurate.