I think people are underestimating TGS. There have been two major shows so far this year (E3 and gamescom) and both times something significant has been shown for GT5. TGS is the last major game show of the year and the last major game show before GT5 is released. It would be very surprising if they didn't show anything new there. And they still have a lot of stuff that they can show that we haven't seen like cars, tracks, weather and the revamped GTTV.
I don't think many people are underestimating it. Well, except the poster above you.
You're right when you say each of the two major conventions have provided us with their own unique information.
E3 2010: Day/night transitioning/ Standard car video, New trailer, new cars to GT5 (Maserati GranTurismo S, Alfa 8C, Lamborghini Murciélago LP640, Tesla Roadster, etc, among others), amazing visual effects video (roll over among other things), Online information, reverse lights shown, etc.
Gamescom 2010: Course Editor, Karting, Skidmarks, new cars (Lamborghini Miura, Jaguar XJ13, Pagani Zonda r, Ford Mark IV, etc). New track (Monza), A Spec/B Spec explained. GT Life explained (to a degree). Tuning options (tires, carbon hoods, etc).
Tokyo Game Show 2010 (in my opinion): Weather in demo or shown in trailer. Full damage in demo. More GT Life info., Customization revealed on the GT site. New Japnese cars (Lexus LF-A, Honda HSV, Honda CR-Z) New track(s). Surprises.