You know, if you're willing to skip out on Arsenal's unique properties and have the cash normal weapons are simply the way to go.
Since Weapons have no cap, they simply are designed for X creature, you can create a Colossal+++ weapon for a mech to wield. Base cost, Weight, and HP all double per Size modification so upgrading to Large, Huge, Gargantuan, Colossal, Colossal+ (mech large), Colossal++ (mech huge), & Colossal+++ (mech gargantuan) creates a x128 modifier with +7 damage upgrades (+8 if you buy your mech some strongarm bracers).
Most WSA become redundant, like adding Flaming to a Gargantuan Mech Greatsword is a +0.04% increase in damage but Splitting is very potent. Hank's Energy Bow, works out to 8k for the +2, 100gp for the composite, 300gp for masterwork, leaving 14.2k for the unique trait. Upgrading this to a +2 Splitting Colossal+++ Energy Bow in turn produces a total cost of 77,200gp, if Size chances are respected it deals 24d6+StrMod+PowerAttack Force damage which completely bypasses Damage Reduction and you get at least six attacks per turn. @30 Str no-PA that's 564 damage (which isn't even enough to sunder it's self).
It's enough you'll outpace even my expected lv14 damage and come on par with Andomander's current. This might be an easy solution for some people who are struggling in the DPR department.