and arguably the worlds greatest ever N/A supercar, the McLaren F1, is powered by a bespoke BMW Motorsport designed V12. of course that arrangement predates the McLaren/Mercedes affiliation, but i'm sure it grates on the M-B top brass anyway...
not to mention the fact the era when McLaren (and Williams) used to trample all over Ferrari (mid 80's to 90's), the cars sported Honda power. Honda should have just stuck with what they did best.