Like I say, I have been using the YN-ST-E3 for ages, AF assist and SC. Plus a fraction of the price.
http://www.ebay.co.uk/itm/Yongnuo-YN-E3-RT-Speedlite-Transmitter-for-Canon-600EX-RT-Yongnuo-YN-600EX-RT-/161173758804?hash=item2586b46354:g:~pwAAOxyUrZSqm0Q
http://www.ebay.co.uk/itm/Yongnuo-YN-E3-RT-Speedlite-Transmitter-for-Canon-600EX-RT-Yongnuo-YN-600EX-RT-/161173758804?hash=item2586b46354:g:~pwAAOxyUrZSqm0Q
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