I've already done it on my alliance elemental. It's the best thing you can do... its still not "great". You are just a stationary gun turret that hopes you dont get targeted. I used Haste/Mastery on my alliance, just convinced crit is better... mainly because even if you use haste, you want savant (mastery) gear more... so your haste is only like 18% tops... rather have the crit so EarthShock has a better chance to melt somebody.
You lose about 10% mastery when you join a BG which sucks , but Overload is ALL THAT MATTERS for elemental, so its that or you just do junk damage... you can stack all the intelligence you want - mastery is just better, two lavaburst crits > one high intel one... plus you are building malestrom faster.
Tho I'm not convinced with the numbers... I have other crap gear that I can get mastery to 101% on elemental... which should ALWAYS proc two bolts/bursts in a dungeon/non BG area... guess what... it does not... so there is some Blizzard Math going on there.