I'm not sure if it's been done before or not, but on alot of defense maps I play, I noticed that as you increase/decrease the game speed, the spawns become farther/closer to each other. I am reasonably certain this is because a wait command or triggered timer is being used. Again, not sure if it's been said, but if you made a set of two locations with your desired unit spacing, and made a unit go from A to B and every time it reached B Spawn one and move back to A, it would keep a uniform spacing regardless of game speed. Also, to increase/decrease the difficulty, simply use a shorter/longer distance between A and B which would accelerate/decelerate the spawning.
Appologies if repeat.
I actually used this in one of my maps, Planets, but in my map, you got 1 gas every time theunit (reaver) reached location B.