The Alpina B6 actually pulls away from a BMW M6 (F13) initially so I don't doubt the numbers. However the B7 LCI seems to be a dud especially at $140k against a S8 (which I happened to order btw). I think the weight was the issue not the engine. I doubt we will see the B3 since BMW does not want to risk losing sales to the new M3. Alpina does not have the dealer network to do it alone in the US.
2013 BMW Alpina B7x SWB Alpina Blue, Dinantronics Stage 2 600hp, 612lb/ft
2016 BMW X5M Long Beach Blue, M Perf parts, RKP diffuser, Dinantronics Stage 1
awaiting 2017 M770i (to replace the B7)