Referring to A: from my point of view a GT86 is not a high performance sports car but an entry level sports car. And a Lexus is not exactly a Toyota, as we probably agree. As you might get from my hint to the Le Mans win, I am trying to understand the Toyota brand strategy here, which brings me to B: why - because they can - or could, at least. They could have gone for a V6 or something else they actually build themselves. Of course, it's a great deal for BMW, no doubt about that.
Needless to say, these are just my thoughts. But brands and their cars struggle to have some profile of their own nowadays. And I don't like the fact that everything increasingly gets mixed up in those cars that are anything but cheap: Supra's with BMW engines, Aston Martin's with AMG engines...