For the reference of future Mecha Modders, I though it would be useful if we discuss exactly how each faction would most likely customize their mechas. I'll start with some stray ideas, and my fellow forum goers are encouraged to jump in with corrections, comments, and additional ideas.
Corporate Faction Customizations:
Each corporation has its own style, but they all share some similar concepts towards customization.
1) Limited Missiles: Each fired missile costs the company money, so Corporate conversions would tend towards reducing costs by limiting missile loadout from the standard design. This brings up the next idea
2) Replace ammo weapons with lasers/melee: Weapons that do not cost money per shot are preferable to those that are ammo-dependent. Of course, the corporation will not put a 1,000,000 Laser Weapon on a 2,000,000 Mecha, but cheap lasers are preferable to cheap cannons.
3) High damage, single shot weapons: Weapons with burst are to be avoided, as they ruin the quality of salvage, which can be converted into raw materials for mecha manufacturing (different than the way that the player recovers and fits mecha components, the corporate factories can use far more than the player)
4) Move the cockpit to the head: A lance of head cockpit mechas will perform better on average than a lance of chest cockpitted mechas. Other factions avoid doing this because mecha are easier to replace than pilots. While true, the corporations know that mechas are more expensive than pilots, especially considering the corporate uses for pilots are probably for lesser-experienced pilots anyways. After all, the corporations are generally the only faction that do not require experience for the player to join.
I'll cover a few of the other factions in far less detail:
Rishi Spinner forces: They generally go for firepower over all else
Pro Duelist League: Increased performance, putting the cockpit in the head, less armor for more maneuverability
L5 law/Silver Knights: Status effects seem prevalent.
L5 Law specifically: Self-reliant mechas that are recon capiable. Upgunned recon mechas especially, L5 Law probably get more search-and-destroy missions than any other faction.