Well, by the name, I would imagine that an ULTRA LBX 10 would be broken, because Ultra and LBX ACs are mutually exclusive.

You're misinterpreting what I'm saying I think. I'm talking about one-shot kills because they translated the damage and armor straight over from the table top without including the random hit locations. The stats from CBT are BALANCED around the fact that your shots hit random locations, with the center torso being the most common location hit at 7/36 shots will hit it, and the head at 1/36. It is this reason that weapons such as Medium Lasers are fine doing 5 damage while a Large Laser does 8, even though it weighs five times more, takes up twice the space, and only has 66% more range.
When they translated these weapon and armor values over to both MW3 and MPBT 3025, they did so without incorporating any extra balance to take into account that weapons fire will not hit random locations in a majority of situations, it will hit the location the player is aiming at. Lasers were the worst because they all hit instantly on the same point. This resulted in the secondary weapons of mechs, medium lasers, being far more powerful than they should have been.
An example anyone who played MPBT 3025 will be familiar with is how brutal a mech like the Jenner was. It mounted 4 Medium Lasers. Each medium laser did 5 damage, so with an alpha strike it would inflict 20 damage. With a single alpha this mech could one-shot any mech its weight or below. Two shots and it would annihilate a 50 tonner or below. 3 hits and Assault mechs were in trouble. Mind you, this mech moved upwards of 118 kph, so even mechs with a range advantage of PPCs or AC/2s or 5s or LRMs were not going to keep that advantage for very long. Often, in the time it took for the Jenner to close the range, the enemy would get maybe 2 shots off (potentially 10 with the AC/2), and then the Jenner would equal, and then exceed that damage in a matter of 3 seconds. The mechs proved way more fragile than they were in CBT. If I wanted one shot kills, I'd play Heavy Gear.
I do have to admit though, MPBT 3025 was a lot more fun than MW3 because of two factors. 3025 IS technology and fixed variants.