Press 2 for Survive-ability and 4 for DPS
Can replace [Stretch] for any other mod, its basically a free slot that works only on 1 and 3 skills which are usually unused.
Hydraulics to navigate the infested vaults/tunnels without needing to eject and re-summon your Mech due to inability to navigate some terrain,
[Enemy Radar] to see what you're shooting at (those infested have really good camouflage at times).
[Vitality] + [Steel Fiber] + Repair + [Redirection] for survival, can remove [Redirection] if the mod slot is needed.
[Streamline], [Flow] and [Continuity] help maintain 4 longer without it eating too much energy.
Intensify works on all skills, even on 2
And the most questionable mod - [Rage], to be honest I don't think it works much, I've been running both with, and without it, and saw no difference, so another free slot if we get something better to put there.