Although I think the damage is absolutely fine - and so is the heat - I think there could be a slight mechanic added to them, which basically only hurts if you really mass SRMs.
Add smoke trails to SRMs!
The more you fire SRMs, the thicker the cloud. For example, a barrage of 6x SRM6 would cause such a cloud, that you could not see through it for a second or two. Naturally the puff of smoke is also "warm" so you cannot see through it with thermal either. This allows the target to move a bit, before the aggressor sees where the target is, adding a new skill based game mechanics that can be used defensively as well.
Edited by Artifex 28, 13 December 2012 - 07:52 PM.