Your statement about rebuilding the Kintaro as 3x SRM6 + 1x SRM4 +++ is exactly the reason (though your damage numbers are off since the +++SRMs do 12/missile giving you 22*12 = 264dmg). Dealing 12pts/missile + 6stab/missile is what makes them OP. SRM6+++ == 72dmg/36stb for 12 heat, 2 crits, and 3 tons (+1crit/ton of ammo).
Does anything else come close to that efficiency? Not even. The closest you get is the AC/20, but that is 5 shots / ton (compared to 8), weighs more, takes up more crit slots, and generates more heat. Yes, the SRMs are "theoretically" spread over the target, but in practice, it's not really that spread and it's easy to score multiple hits on the same hitbox (including the head) just with good positioning (side shots are only going to be targeting 5 locations).
The Kintaro can go as high as SRM24 by doing 2x SRM6, 3x SRM4. And if all of them are +++, you're looking at 288 dmg. For a whopping 12 tons of weapon systems in 5 hardpoint locations and consuming 7 crit slots. A single AC20 is 14 tons and does 120 (for the +++ model).
(Assuming max damage+ models) Compare that to the PPC (60dmg @ 35 heat) or Laser Laser (50dmg @ 18 heat) and you're better off with SRMs with the engagement ranges what they currently are. For Autocannons, you need too much tonnage for the amount of damage they do (though the heat is their one saving grace). Medium lasers are the only thing that holds a candle to SRMs, but even there you're only looking at 35 dmg @ 12 heat for 1 ton (and no stability damage) and for 5 crit slots, you're topping out at 175 damage, not 288.
I will have to take a screen grab and figure out how to put the dmg on here.
There are two M LASER on the KTO as well - they do 50 dmg. 3 x SRM6+++ = 216 + 48 (SRM4+++) for 316. I HAD 2x M LASER+++ for 70 dmg (336) but I moved them elsewhere.
And the stability damage. And the TTS++ for accuracy.
I think that is the most damage one can do really? The Grasshopper can do 305 all laser with +++ 7 M LASER and 2 S LASER+++. Closest I can get.
Well non-modded/alt 'Mech variants anyways....