Yes, an assault MECH is wasting it's armour hiding in the rear. Probably better for your train to run a heavy or a medium. Or not at all...
"The distinguishing characteristic of the lizard-brain is primarily the conservation of energy. Instead of pushing its stuff around all over the place, the lizard puts its stuff in one place and stays there. It only picks up and moves when it sees something useful and desirable, or when something distressing and undesirable shows up."
Here is how I run my STK-5M. Please note that because the mech is usually out of the general fray of action that an XL is used to allow more weight for ammo and the ARMOR STRUCTURE of the skills tree is ignored allowing more points in WEAPONS and SENSORS.
NOTE: The two MISSILE RACK +1 settings in the WEAPONS tree are selected. Even though there is only 11 tons of ammo, normally 1980 missiles, this tree option uses 2 points and increases total missile load to 2376. SENSOR tree gives full Radar Dep, Seismic, and Target Delay, along with a couple other additions. AUXILIARY is set for 2 consumables... a peaked out UAV and a cool shot.
It can run hot on continual chain firing as there is never any cooling time between launches. If so steal 3 points from somewhere (maybe the 3 HEAT GEN points at the bottom of the WEAPONS tree) and add the bottom 3 skill points in AUXILIARY (COOLANT RESERVES, COOLSHOT COOLDOWN, and CONSUMABLE SLOT) for faster cooling and an additional cool shot consumable.
I prefer the 4 ML over the longs as the longs require line of site (making you visible), use too much weight, make you run hotter, and they point right at your location. Also, I don't think 3 15's is enough. If you are going to build an LRM boat make it a battle ship.
A 1000 damage is not unusual with this build with the right potatoes on the battle field. I hope this gives you a reasonable starting point for how you play LRM boats.
EDIT: You mention "ammo for days". On numerous occasions, when run as a pure boat and just continually raining on people, I have run out of ammo... all 2376 of them.
I've determined that both ELO and PSR are myths, weight class matching a fantasy, hit registration a dream, and unbalanced teams a goal. The Matchmaker algorithm is entirely a function of pure spite and malice. And if something actually works they will fix it until they break it.