Just my thoughts, and some are wordy. Hey, I dig the technical nature of the game!
I'm in the camp that AC's should have an impact event, along with splash damage (HEAP = High Explosive Armor Piercing). I also think that it should be a chain burst of rounds that cause an impact event on the person firing it as well, scaling with the size of the AC.
Gauss should have an impact event, but much smaller than the AC. It should not have splash damage.
PPC's should cause a heat transfer along with EMP effects, but only a small impact event. You're firing a pulse of ions. They have mass, but minor compared to a slug and in turn have greater distances with the benefit of electrical transfer.
LRM's should have a minor damage/impact event below minimum arming range. The impact event above this range seems balanced to me now.
In regards to lasers, I don't feel the benefit of the smaller lasers in regards to DPS and tonnages. Obviously, the longer the range the more energy required to reach it (Due to particulates dispersing the beam). In turn, it takes longer to charge the capacitors required to fire the laser. This results in more damage over a longer distance, but a much slower recharge time, with much greater heat. The amount of energy increase required should be logarithmic, not linear, right? Also, as the air is super-heated from the beam, it's resistance drops so the damage should also be non-scalar the longer you hold the beam on a person as opposed to a steady tic-per-second as it is now. Think of how lightening works with a much smaller charge (fingers) "paving" the way for the larger current. Therefore the end of the pulse should be causing more damage than the initial tic.
So, given this paradigm, speaking entirely to DPS (with the factor of heat), a mech with SBL's should smoke a mech with an equal tonnage of LBL's at 200m. The extra tonnage does not transfer directly to damage the same way a cannon can't simply be loaded with twice the powder, the extra weight is due to the need for stronger components.
To summarize a wordy explanation, 4 SBL's != 2 MBL's != 1 LBL. I realize this is limited by the canon, but should be addressed for the mech lab. It's always been a common practice to fill your mech with the largest weapons possible then stuff in the smaller ones to fill the spare tonnage. I'm not sure that should be the most efficient way to design your mech, as in theory (assuming you can mount them) if my desired range is 500m then MBL's should be a far better choice than LBL's...currently it doesn't feel so.
Heat, overall, doesn't have a big enough impact imo. I always felt from the books that the choice to use an Alpha Strike was a heavy one, as it comes with sizable consequences in the form of heat. Right now, going over the shutdown threshold doesn't seem to really have a penalty as long as you can avoid the shutdown with coolant. I'm told it can damage your mech but I really never notice it in practice. Perhaps some kind of DoT effect based on your current heat level so if you ride near the shutdown line you're taking small but constant damage? And any spike over it should have a sizable damage penalty to the core of your mech.
Hey, I could be full of crap...but it sounds convincing! Enjoy the discussion!

- Bogeyman