The 911 will (just) fit four adults and has a descent size boot, does that make the RS6 a 911 rival? In which case why make the R8. My old Celica would fit four adults (without the need to remove legs) and had a huge boot does that make the Ford Focus a direct competitor?
I would personally say no, the RS6 will be a lot more comfortable in the back that either the 911 or the GT-R and I would guess carry quite a bit more. And if early road tests of the GT-R are correct it has a suspension set-up that is only slightly more compliant that an R34, if that is correct then in no way could the word cruiser be used. I quite like my fillings and having driven an R34 and an RS4 (nearest I can do to the RS6) I can assure you that a long distance would be much better served by the Audi.
If the GT-R has more usable rear seats and more luggage space than the 911 that gives it a potential selling point over the 911, its quote a leap to then say its starting to become a direct rival to the RS6.
They are aimed at totally different parts of the market, they are aimed at (estimated) different price points and almost certainly at different 'lifestyle' groups from a marketing and brand point of view.
I honestly don't see many GT-R customers standing next to the car and saying, now I know I want something that shouts stupidly aggressive coupe, so I will go and buy a saloon/estate car. I could however see them saying, actually I'm going to beg, borrow and steal the difference and go for an R8 (otherwise why do Audi make the R8 - if as has now been claimed the RS6 is a competitor to the GT-R which is targeted at the 911).
Regards
Scaff