well ive had 2k units on screen with only moderate lag, that was without colision, shadows, occlusion, lowest quality settings (footmen), and just sitting there. with a pretty fast computer though, and it didnt have any other running apps, except those required to run windows. Colision system makes quite a bit of lag for the game, ive noticed. Just so you know colision is what keeps the units from walking right over eachother, walls, buildings, ect, so an artificial system would have to be administered to replace it (i use unit enters rect, and then lace every wall with it. for units i use a buff
and by moderate lag i mean, it was playable, and i got about 20 fps