I'm not sure if it is highly unlikely, I've read both sides of the argument to why they would and wouldn't release it early, with a Schizophrenic company like Sony, either seems just as possible.
One last Hurrah for the PS3 before they begin on the PS4's GT7 might not be such a bad thing, the physics are already pretty good, if it has enough new content too, then it could be worth while.
At the same time, being so late in the year, who knows, we might be witnessing a launch title for the PS4 as well, either is technically possible, and I'd prefer it as a launch title honestly.