I have...
Just tell people that want reduced build times to go with the
Gem Boosted Builder ideas. It is even listed in the sticky thread as a possible idea that maybe added. Not to be confused with the multiple Builders on one building idea, which is ruled out (aka, 99% chance of not happening). The Boosted Builder idea is much simpler and makes more people happy, including Supercell. Trading and balancing that, and making new code for it just isn't worth it when the code for boosting is already there, and just needs to be implemented for Builders if Supercell ever gets to finding balanced cost values for boosting.