I think it's just dungeon quest xp that is being reduced, mob xp should remain the same so it shouldn't make a huge difference really.
The last toon I levelled was my shammy, around 3 months ago, so things have changed some since then, but you should still be able to level faster grinding instances than from questing, especially if you're a tanking or healing class.
You're obviously going to lose out on a whole boat-load of new and improved content this way though, so I really wouldn't recommend it unless you're planning to level another toon when Catacalysm goes live. Instance grinding is faster in my experience - not so much for dps toons though - but it's not like quest levelling is going to take you a whole lot longer, and it's a lot more varied and enjoyable besides.