If you're looking for fastest xp possible, of course get all heirlooms. You'll never find a better shoulder/chest while leveling.
However, a rare-equivalent weapon at all levels is a pretty huge buff. I'd say it is better than 10% exp rate, as it makes all quests which involve killing stuff trivial. For the majority of the game, your second most important stat is your weapon damage/speed. (Most important: movement speed)
The reason I wouldn't grind out shoulder/chest heirlooms now -- aside from the fact that things are relatively overpriced in JP -- is that those won't materially affect the enjoyment of 1-60, which is quite well done now. But making the content easier with a fat beatstick definitely helps, as there are still some quests that are a drag.
And if you are on a PvP server, a weapon is a damn good item to play with in any contested zones. Some dumbass frost mage @58 attacked my druid @58 yesterday, and proceed to get a face full of 3k dmg crit ravage; he was dead before my first mangle landed.
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