We've just heard from Canon, over on their own forum, that they have acknowledged the problem, and a fireware fix should be in the pipeline 
http://forums.usa.canon.com/t5/EOS/5D3-AF-assist-beam-slower-focus/m-p/13107#M1450
Just relaying the news.
I'm now a very happy 5D3 owner...
[mod edit: changed title to reflect Canon's statement that the fix applies to the 1D X as well]
http://forums.usa.canon.com/t5/EOS/5D3-AF-assist-beam-slower-focus/m-p/13107#M1450
Just relaying the news.
I'm now a very happy 5D3 owner...
[mod edit: changed title to reflect Canon's statement that the fix applies to the 1D X as well]