The problem with more lances is that a given mission will take SO much longer to slog thru.
When you are facing two or more lances of enemy mechs and have ONLY 3 allied AI mechs on your side, the amount of time it takes for a round to complete can be excessive, as is. Adding in more targets for the AI to calculate around is only going to drastically increase the wait time.
The game, while certainly fun, is already excessively slow. It's even SLOWER with some of the features you talk about being added via the unsupported mods, like Battletech Advanced 3016, which includes AI tweaks, as well as some of the other elements you talk about, like fusion core breaches, etc., etc. There are some missions I've done with that, where the AI can take upwards of 20 seconds to work out all the paths and options, before moving a single mech... then it goes through the same algorithm again. The AI changes, sometimes, are really awesome and challenging, other times? The AI just acts really dumb, moving into a prime position and firing no weapons, even though it has no heat, or just running, making no attacks, just to scoot back closer the next turn, also making no attacks.
It's a complex game, adding more lances to the field, even with the base game, with the base AI, is going to really bog it down.