The other thing is that most of the xp/tp you get will come from quests instead of grinding mobs. If you like grinding mobs better, then yes you will gain a lot more xp/tp from killing high level mobs, so in that case you should also try to level up. Recently, I was using my lvl. 70 class to kill lvl 64 mobs and was gaining around 300 xp per mob, just to give you some numbers.
300 XP per mob - proportionately, that's probably even worse than the 10 xp per mob I've been known to get when cleaning up a bypassed quest. (Or 0 XP< I think if the mob is > 10 levels below me, or if I'm partied with someone >10 levels above the mob.) I get 300+ per mob at level 35 or so, at least if the mob is level 38ish. (I can't give exact numbers right now, since I've now hit 36 and haven't found any level 40ish mobs to play with ;-) Also, I think the bonus is already in effect.)
This is an example of where I don't want to be - what do you get when killing level 74 mobs (if there are any)?
I know I've leveled on pure combat XP on occasion, without turning in any quests, and while it wasn't as fast as using quests as well, it wasn't all that slow. But they were "big" mobs, the kind that could kill me if I slipped up, or got mobbed.