I'd say TT is 65-80% GT4. Tracks are the same, and that's a fair amout of the game to be sure, but not much else. The menus are similar I guess.
GTPSP may be a downgraded GT4, but it surely took a fair amount of effort getting it on the PSP.
GT:HD... yeah, it's pretty simple. New track to be used in GT5, with the rest being GT4 resources.
And yeah, GT5P is simply a sample of what PD has been doing with GT5.
Just a note:
GTHD was planned to be a whole release of a GT game that was suppose to go by the way of small transactions and such. In the end, the whole project was obviously scrapped. I think it may be due to the number of complains people showed about it. The planning of this had to take some consider amount of time.
Then after GTHD, they moved on to a new project; GT5P. This was the beginning for the real GT5 (new physics engine, graphics, cockpit, etc.). However, they had to concentrate specifically for a design that is suitable for the game itself (not GT5). Then localization, advertisement, and specific and compatible updates for the Prologue was to be made. I also assume during this stage they had a hard time getting along with manafacturers in terms of damage (perhaps this is why it was canceled).
Then somewhere between the release of GT5P in Japan and the Spec III update, they probably started working on specifically for a GT5 interface (new physics, graphics, online, etc.). Then somewhere in the summer, GT5P development is over. Full GT5 time?.....
Not quiet yet!
Then out of nowhere really, GTPSP development chipped in during those times. I'm not too sure about its affect on GT5's development, but they did say at one time they had to concentrate 100% on it, so perhaps it did take some valuable time. Achieving 60fps as some of you said, was an experience that was totally new to them, and something that was learned in the process.
So really, I think all those stuff did take a big chunk of time off, compared to if they just started working on GT5 alone from the start; one plan, one project.