You can fire all weapon without over heat in terra terma. But next my advise is to use the AC20 and chain fire of PPC to finish your target. Stay over 200m of your target.
The 250 puts you 'round the speed of an Atlas, so that's not hideous. The two MLases in the arms feel superfluous, though... but .5 tons or armor plus AMS seems even more pointless. Whatever it takes to get your drop tonnage down, I guess.
I feel like it would be worth dropping the engine some just for the fact that you only have 12 heatsinks in that thing. 2 PPCs and an AC20 is going to cook that thing.
This is also an option, using a STD 280 I use in the ON1-K. Squeeze an extra heatsink in there, and 60 kph is still pretty respectable for an Orion. Added 2 MLas just because I feel that this setup is just asking to get killed by a light mech. I dislike how much armor had to be sacrificed, maybe someone can elaborate a bit.