The way I see it, a level 50 could sit down and churn out low/mid level HQ gear/weapons, but considering how much gear is thrown at you during quests (so much that in the end you just start taking the gil) that'd be pretty silly.

The only thing I can think of which should prevent Legacy from transferring to non-Legacy is PVP. A fully decked out Legacy char will dominate PVP on that server up until the time the rest of the server can catch up with them gear wise. Actually having Legacy players for PVE would be quite helpful, looking for a decked out tank to take on Insane Mode Moogle Killing Battle Arena? Keep your fingers crossed DF pairs you up with a decked out tank from a Legacy server and not a new player using level 12 subligar because it makes his butt look nice (though personally I'd tank dressed up as a reindeer).