I believe that mechs equipped with fully articulated arms shouldn't be penalized by having to expose large parts of their mechs just to clear the ground or another obstacle for their low slung arm mounted weapons.They should be able to raise their arms forward to shoulder level on demand and fire over obstacles.
What about a toggle that moves arms between two positions?
First would be the current position with weapons placed low and a shield arm effect protecting both side torsos.
The second would be a raised position with arms raised forward (like a zombie

). A mech would be better at hillhumping, but his side torsos would be more exposed to flanking fire.
A mech would require both lower and upper actuators to be able to use this function.