Kris, it has been mostly GaN on Sapphire so far because the epitaxy buffer layer AlN and the active layer InGaN have so far been processed mostly on Sapphire. That is not the only substrate of choice, there is bulk GaN, SiC (early Cree versions), Si and composites.
If yield issues are proven, it makes sense to move away from Sapphire. Ideally, the LED industry has to work toward vertical designs with backside contact which can result in smaller chips. It is not clear if moving to Si substrate achieves that.
It is ironic, Toshiba on one end announced cut in its NAND Flash production by 30%.