This might sound good to some, but it would have a negative impact on syncked dungeon runs in that it will only exacerbate the divide between players of different skill levels.
I'd like to go back to this point from page 1 for a moment. dseth's right; if we had access to all our real-level actions at syncked levels, they would absolutely need to be scaled down to the point that a Lv70 player doesn't do significantly more damage than a non-syncked player.
Assume that syncked players don't mind executing their real-level rotations, but at reduced damage output to maintain fairness toward lowbies. Even if this were the case, there would still be a problem because many syncked people in the DF won't execute their rotations properly.
Which brings us back to
We don't need people practicing their real-level rotations in syncked dungeons, because they'll be executing their incorrect rotations at reduced potency, which will make runs drag. It's all well and good when the syncked people are both able and willing to do their full rotations, but let's be honest: given appropriate scaling described above, there will be many syncked DF players who can't put out as much damage with their complete but throttled action sets as the lowbies can with their incomplete action sets but unthrottled damage.