particularly when chipped to 'only' ~500HP, and even with a small amount of turbo lag methinks the new RS6 with 580HP will feel like an absolute rocket vs. the current S6.

Just a guess, but I'll bet the difference will feel similar to a current S4 (high revving 'racey' V8) vs. a modified B5 S4 (massive torque & much faster than the current S4).