A friend and I have double TRed over this bonus XP weekend, we decided it was high time to see everything in the game, no more skipping content for the fastest route (though it's not exactly been slow), so here's what we did:
Everything in korthos, normal, hard then elite.
After we hit the harbour we cycled everything,
level 2 quests on normal,
then level 3 quests on normal/level 2 quests on hard,
then level 4 normal, level 3 hard, level 2 elite and so on
By the time we'd done level 4 normal/level 2 elite and half of level 3 on hard we literally had to stop holding level 6 and just take -10% on the remaining level 3 hard quests, we've done about 6 level 5 quests on normal (no level 4 hards/level 3 elites done yet) and are at rank 33, we're going to have to stop holding level 7 soon.
I realise we're currently gaining 25% bonus xp, though even if we weren't we'd still be far ahead (we'd not have lost so many 10%s on hard level 3 quests for one thing).
It's kind of interesting that nobody seems to do this, while it definitely isn't the quickest way, and we will farm select extremely high XP quests we don't seem to be struggling for XP in the slightest, indeed there's a ridiculous abundance of available XP. Plus we don't have to come back and do the favour grind later, plus it's less tedious than farming the same quests over and over, it turns out that many quests give FAR more xp than I ever realised, even before the bonus 25%...