Well I just don't think 1500 unit cap is much of a barrier, especially if you have it shared with allies on death. I don't think I've ever hit unit cap in those situations; only where unit cap is not shared, or it is only 1k cap.
But one slight problem is if someone has to go afk and walks their acu to a safe spot and just transfers all their units, thinking they might be able to return in 5-10 minutes, but give everything over to avoid inconveniencing everyone. In that case, a solution would be to have 1500 unit cap per player, available to the whole team.
But besides that tiny change which impacts maybe 1% or less of my games, I don't think I'd want a higher unit cap. The game just isn't optimized well enough, so pathfinding issues really crop up well before we would be hitting 6k units for one team in a 4v4. It frequently happens that if you give new orders to units they just lose their previous orders and stop moving, necessitating a shift-g to try to get them to move. I have seen it many times before on setons. I'd rather play games that have at least some incentive to avoid that cancerous, unenjoyable -4 sim speed pain for everyone involved.
One thing I'm not sure about is how buildings affect lag. Maybe we could have buildings not impact the unit cap? If that would be the case maybe just a 1k UNIT cap would be plenty.