Well, I partly agree with you. The amount of data collected for GT4 was extensive, more than the PS2 could use, and I've been saying for a while now, based on things Kazunori said, that they built a much larger GT4 than we got. I think that work on GT5 has been going on for a few years now.
As for the engine itself... that depends. Sony has been mentioning for a few years some tidbits about Cell and PS3, as far back as 2001. Polyphony may have had a bunch of details about the specs and architecture, enough to do preliminary work on a GT5 engine. But keep in mind that the most effective programming is done when you know what you're coding for, and the final build of the PS3 is a pretty wild beast, with eight processors all running at 3.2ghz.
It's possible that the GT5 engine was so good that all it needed was tweaks to get it to fit this multithreaded beast. But I'm thinking that building it from the ground up for the hardware is the way to go, because that's a seriously different chipset in the PS3. Divvying up different aspects to the SPEs to handle like bot A.I., physics, weather effects, crowd behavior and so on would have to be coded from the start or it would be hard to reconfigure for something like that, I'd think. We'll certainly see. Everyone who deals with Cell and RSX is in some kind of euphoria right now, and preliminary work done on games look at least as good as second gen 360 games.
I'm excited, and the future can't get here fast enough.