So I'm not sure if you've ever done a gem boost or what not. But the way it works is if the spell factory is boosted, when you queue say a lightning spell the time is listed at the normal 30 minutes. However, it counts down 4x as fast.
So the idea is that as you switch from league to league the rate at which the time counts down changes. That means there would be little advantage to starting a building in Masters and then dropping down to gold because you would only receive the masters advantage while you were actually in masters league.
It's true, though, that this would make determining exactly when a building would finish a bit difficult. That is definitely a potential issue with the idea. Perhaps this could be solved with the info screen which could list the current time, the multiplier, and the actual time left if you were to stay in that league.
If applied to troops though, I think it would be less of an issue.

