I've only driven an R/T Classic and 392 SRT with a 6 spd so I can't compare either against and automatic, but I can tell you my 392 was Hella fast as set up, or even stock for that matter. The only auto Challenger I've ever driven was a V6 rental while my SRT was in the shop. Between the drop in HP and the lack of a std shift, it was pure torture!
Now I'm ready for a 2015 and I've heard so many good things about the 8 spd auto transmission, I'm torn between which one to order. I think I'll really need to test drive one before making the decision. I was able to turn the rental back in after a few days, but I'll have to live with this decision for a lot longer and I just can't picture myself NOT shifting manually.
But anyway, back on topic, the 392 with 6 spd is plenty fast if you know how to use it.