I think GT6 probably could come on PS3. Why?
You are talking that GT5 is already stretching PS3 and GT6 will have to come out on PS4. Yes, but look at GT1 to GT2 and GT3 to GT4. I have played only GT2 and GT5 (Prologue too), but I've read a bit about this. GT2 in comparison to GT1 wasn't really any better in graphics (smoke effects was different). Some said that it was even a little bit worse. GT4 and GT3? Again the same, very similiar, some say that GT3 looks better. So don't expect GT6 to be really much better in terms of graphics than GT5 (if it'll come out on PS3). GT2 and GT4 had just much more content than GT1 and GT3.
Personally, I'll be glad if they get the shadows better (less jaggies, less flickering) and I will be happy if they give us choice between 720p and 1080p. Real choice I mean - 720p has less flickering shadows or something like that. Because now 720p is just less blurry on textures (MSAA instead of QAA, if I'm right), even I can see it on my native 1080 22' monitor. But flickering is still there
So, in conclusion GT6 will have more content, but same graphics, as it was with GT4 and GT2.