I have Toyo Proxes on the other wheels. I had the tire place check the rear tire wear and they were at 4/8 - 5/8. He also measured new tires and found that they were 10/8. That means that they are about half the tread.
My understanding is that, that is too much difference and will be bad in the long run. I think that by saving $300 now would cost me more in the long run.