I think the person who is interested in an ///M5 and the person who is interested in a GT-R is not usually the same person
The driver of a GTR could not care less about the luxury, space, practicality, and Q-car status of the ///M5, whereas the BMW driver values such comforts above the pure performance aspect of the GTR.
Also, to me the GTR, although a technological tour de force, looks like a squared off cheese grater.
And for a two ton sedan to perform the way the M5 does in this video is remarkable.
If you don't directly compare the two, which you shouldn't, as they're two completely different types of car, it does smashingly. Granted, it loses according to Sutcliffe, but to be as close to the GTR as it is, with all the extra weight and comfort features...damn BMW, you've done it again.