As much as I'd like to say "this is what works" these kinds of battle games can be difficult to make work. You're right about bullet to armour ratios, in a technical side anyway, because most armour today (and going by advancements for sci fi purposes) would survive only a handful of rounds before critical weaknesses would cause it to be as effective as tissue paper shields in a warzone. Missiles, depending on the type used, could easily penetrate even heavy armour with a single well placed shot. Same goes for certain cannon rounds, such as the HE or Sabot, but again this is difficult to gauge for a 100% success ratio.

If I may be so bold as to say, it might be worth moving this style of RP to a more Dungeons and Dragons setting. I've played a lot of D&D in the past and I can say it works pretty well. The issue will be that our mechs will need to be set into a strict guideline system which will be heavily effected by numbers. Some may need re-working altogether.

If you decide on the Dice roll idea Titan, let me know and I can find my Stars Without Number guide lines a friend gave me in PDF. format. It's a rather confusing book at first, but if you guys are at all interested, the option is there. I know a roll site too and, if anyone is interested, I even have Star Wars rule books too. These styles of games can be a hell of a lot of fun.
I DM'd (Dungeon Mastered) a Clone Wars game once. Had a situation when one person stole a tank and used it to blow up a munitions depot. Only issue was another player was in said depot...he lived.