Ok like exo said, the DPS LMG is so not worth it on tortos bat tanking, infact you will not even notice it, the proc always hits your current target, and as far as i know even if you hit 10bats with thrash you will only get 1 charge.
BUT!!!, if you are tanking the bats they tank lmg is so far superior to the LMG gem. You will mitigate ALOT of dmg with that, cause it proccs more often while aoe tanking. The stomp from tortos is Physical dmg(this is what kills you in combination with the bats).
@exo: I agree on 25m i would take the tankLMG aswell, the raiddps gain may be very low. I was talking about 10m in my last post. Megaera is a great example with the way you execute your fight, on why the tanglmg is better if you dont kill the green head.
On all the other fights melee attacks will not kill you and your healers should never have problems healing that (not talking about durumu when you have 8 healingdebuffs).
In 25m i would say Tanklmg is the better choice, in 10m the extra dmg and not all bosses hitting so hard (tortos and magaera if you dont kill 1 head the entire fight excluded) the dps meta is a bit better.