Terrible logic. T3 mobile arty only has 0,08 dps/mass ratio, so only about 25% of T2. Do you want to quadruple its damage after your MML buffs (applying the same logic as you did for your MML buff)? Then it would still only have half the effectiveness of your T2 MMLs. Cyb T3 mobile arty would then deal 1800 damage per shot. Only 2 of them (1600 mass cost) would have the same dps as a T3 artillery installation.
Just an example to show that this way of looking at balance is dumb and ignores many other things, like the additional range you get or that you force your enemy to make a building that is more expensive than an MML and is otherwise completely useless.
In general I think they are pretty good, they outrange T2 PD so can kill them with 0 danger. If your enemy wants to protect them he has to make a shield and/or build TMDs, so he's technically already losing even if you kill nothing. They are decently fast and can disengage easily. They always hit what you shoot at, unlike artillery. I think they fulfill their role well enough.