From everything I've seen and heard, I do believe we will see GT7 on the PS4 IN 12-18 months.
My opinion is that GT6 was originally going to be a PS4 game, yet Sony pushed it back to the PS3. So I believe Polyphony Digital already has Gran Turismo up and running on the PS4 in some shape or form. The GT7 tech demo of inside the cockpit of a car at night in the snow leads me to believe this. So GT6 on PS3 is, IMO, a PS4 GT game downscaled to PS3, so thats why its having some performance issues. I dont believe PD simply bit off more then they could chew with the PS3, they're just having trouble downscaling a PS4 quality game to PS3.
Also theres no way we would have heard Kaz say TWICE now GT7 could appear in a year or two. Obviously he has confident in their ability to produce a game on the system quickly, as the foundation on PS4 is likely already in place.
Why would GT7 on PS4 in 12 months be such a bad thing? Sure, alot of content will be cut. Maybe half the tracks and 75% of the cars. But since when did amount of content make a game? 4-500 cars and 16 or so tracks is enough to make a great game. GT3 had 180 cars and 12 or so tracks, and its still the highest rated GT game to date.
I know the other guys had to cut the Nurburgring, but you have to remember that version of the Ring was terribly inaccurate, and will have to be built again from the ground up. I dont see GT7 having such issues. Just give it 3D trees near the track, smooth out the barriers, and add higher res grass and road textures. They dont have to rescan the whole thing again like the other guys.
Bring on GT7 2014. Why not? Nobody is forcing anyone to buy it, and it gives PD plenty of time to patch problems and produce DLC for GT6.