Currently this is my build. Fast (for a mech assault), durable and with a respectable punch. Originally had an AC20, but changed the GAUSS to give it some damage at long range and improve heat eff.
Quite a unique build. Definitely not an Alpha Striker, but the high heat efficiency allows for sustained fire. If range is your thing, perhaps this alternative: The Alpha remains the same, not that its particularly important, but the range of your back-up weapons is increased by 50%. Heat efficiency is also improved and although the overall armour rating is less, all three torso sections are still fully protected. Also, moving the Gauss ammo to fill the left torso crits reduces the chance of a critical hit striking the vulnerable Gauss Rifle.
I'd personally drop to a STD300. I don't think 10 engine rating is worth 2.5 tons. In its place, you could add another LPL or an AMS or whatever.
