Nah that won't be the case at all. Even T3 gear is comparable in stats (minus stamina) with the first TBC greens. The only thing that "saves" T3 gear are some of the set bonuses, but all of this is gonna be largely useless at lvl70.
The only reason people raided with high end vanilla gear early in TBC was because the itemisation at that point was very bad, however blizz has massively buffed every item in TBC (while nerfing vanilla items and straight up disabling many effects past lvl61). We will with a 99% likelyhood get the buffed TBC items from the start, which will render pretty much all vanilla items useless at lvl70, except for a few select items.