I got an idea. How about giving factories and ACU a slider on UI that ajusts build rate. This could be usefull whe acu is building and experemental say but you want it to build it without stalling mass so you can still produce other units reasonably fast or when you want to tech up mex without stopping facs.
Exactly... or as I do, just move them on to something else. If you have 20 engies on your XP and 10 on ur fac, and you want more units faster, transfer 15 from the XP to the fac.