My 550 xi was underpowered, but the stage 2 Dinan flash puts it about right. It is regrettable that the new M5 isn't offered in AWD, as I would have purchased that in a heart beat. The Dinan stage 2 in a 550xi puts it on par with the new S6 (while saving me about 15 grand). Honestly, the only car I have ever owned that I thought felt spot on power wise was my 07 911 Turbo, curse me for needing something more practical.
Current: 2012 M-Sport 550i X-drive Dinan Stage 2, 1973 911 with 930 Motor (340 hp 2200 lbs).
Gone but not forgotten: 2007 911 Turbo, 2005 911 Carrera S, 07 BMW M Coupe, 07 335 XI, 05 Mercedes R500