Even if the IQ is acceptable, I worry about AF performance. Either the aftermarket just doesn't have good reverse engineers, or Canon has hidden some things that can't be duplicated. If Tammy could really nail the AF, for advanced AF cameras like the 5D3 and 1DX, I think many would accept small compromizes in edge sharpness, maybe a little contrast, and and even bit of CA that could be corrected in post. Its just that when you are out in the wild with a 600, the last thing you want is an AF system that misses some x percent of the time, as you probably won't get that moment back.