I had ushio 14k, then i moved to Phoenix 14k. The difference is amazing.
It was a little too blue at first but after a couple of weeks it was a nice clean light blue which i wanted.
Growth has been very good, nothing miraculous. Color of my corals improved a lot.
From what i've read, under 20k lights, the color doesn't really improve for all corals. It just looks better.
If you have deep water corals, then they will appreciate the lower intensity of the 20ks.
Its basically matching the intensity of the light to the corals natural water depth in the ocean. For some coral photosynthesis happens at very low intensity(zoas), for some it has to be relatively high.
So theoretically, the longer you keep the 14k bulbs, the better your corals will do as far as color. There will be a point where the bulb has deteriorated physically and can no longer yield desired intensity or color, but that should be 12-24 months from the point of installation.
Some growers will use 10k lights initially to improve growth, then switch to 20k for better color. Thats because the 10k lights increase the formation of brown photosynthetic cells. when they put them back in their natural low intensity light, their natural color is restored.