Applying antialiasing at native resolution does little but fuzz up the image slightly. It may be more pleasing to some eyes, but that's an individual reaction.
Just dropping GT6 (on PS3) to 720p isn't going to free up a treasure trove of resources. Rendering that 1280x1080 image to 1920x1080 is just a small part of what the Cell Engine is doing every 60th of a second. It definitely helps, but I think my list of things to cut back is what's going to be needed, and likely more technical things I haven't thought of that are intrinsic to the GT Engine and Cell architecture. PD engineers likely have been working with this over the past few years, and if GT6 does come out on PS3, I'm hoping to see some definite improvements. The ceiling is vastly higher on PS4, so I'd expect monumental improvements on it across the board, graphics aside, which I expect to be like GT5 but flawless.
The one thing I think would be notable graphically on PS4 is something Griffith500 mentioned, and that's amazing realism in metallic, pearl and chrome paints. Plus slightly related, there would be all kinds of processing power available for breeze/wind modeling, moving trees, flags and other items, better spectator animations, etc, that "living world" thing.
One thing about amar212 is that he's something of a SONY/PD insider, and while Microsoft might have let him in on some of Forza 5, he wasn't with Horizons which wasn't even a Turn 10/MS product. I don't think they would open up to someone involved in "the other side." Plus, it seems he was talking about the week before the buig MS luvfest. We'll see in a few weeks though.