To be honest I think Joubert would have been a bigger threat to Plushy under the old 6.0 system then the current one. People are led to believe Joubert is almost unbeatable by this season but none of his main competitors skated great competitions in any of the events he was in. A clean Lambiel would beat him everytime. A clean Takahashi would probably have most times this season even with his current jump content-and if Takahashi adds a quad in the short, and a second in the free, he even more obviously will. A clean Oda could even be competitive because of his level 4 elements, and huge GOEs, even without a quad. Plushenko would still have to have serious mistakes to lose to Joubert. Joubert even relied largely on the failings of his competitors in the Plushenko-less field to dominate this season, more then his invincible skating or anything.
Lambiel with a stabalized triple axel-if that ever happens, Oda with a mastered quad, or Takahashi with a bit of a further upgrade on consistency and adding an addition quad in both short and free, are the only possible threats to Plushenko. Joubert is not a major threat to Plushenko at all, unless he is far down from his former level IMHO. As of now, none of the above has happened, so Plushenko is the runaway favorite if he returns in his old form.