Concerning the Speedlite AF assist beam issue occurring with Speedlites used with a Canon EOS 5D Mark III or Canon EOS-1D X, there will soon be a firmware fix. In the EOS forum at Canon USA, a forum moderator wrote:
Canon has confirmed that, depending upon the shooting conditions, it may take slightly longer for the EOS 5D Mark III and EOS-1D X digital SLR cameras to acquire focus when using the Speedlite’s AF Assist Beam, compared with that of the EOS 5D Mark II and EOS-1D Mark IV digital SLR cameras using the Speedlite’s AF Assist Beam. We are developing a firmware update to enhance the focusing feature for the EOS 5D Mark III and the EOS-1D X. The schedule for release of this firmware update will be announced shortly.
“Announced shortly” could mean in April, when Canon is set to release a firmware update for the EOS 5D Mark III. Don’t think they will release another one before.
Canon EOS 5D Mark III price check: Amazon, B&H, Adorama, Canon USA, J&R, Digitalrev, eBay, rent it at Borrow Lenses
Canon EOS-1D X price check: Amazon, B&H, Adorama, Canon USA, J&R, Digitalrev, eBay, rent it at Borrow Lenses