Interesting idea. I'm more inclined to agree with Zach on the build times. However i really like the same idea applied to the speeding up of barracks / spell factory in higher leagues.
I'm sure Supercell would prefer everyone to raise their trophies rather than low level farming. You can just about farm with tier 2 troops (balloons/minions) in Masters but spells are also needed. It starts getting expensive in resources and build time which eats into loot gained per hour.
We've said it before but if they want people to raise trophies then they should reconsider the ability to use heroes during upgrades. This would greatly help players and also encourage more to upgrade them.

