Heard that questlog gets wiped at a new expansion release?
Nope, I was able to turn in 25 quests when wrath came out for ~250k xp.
It might be worth doing but unless you're gonna serious-face go for realm first, you might just want to not bother since the quests in cata will most likely double the xp you get from wrath quests (probably more)
As long as you can get out ahead of the pack, leveling shouldn't be bad. My biggest concern is staying logged into Mal'Ganis when i'm trying to level.