Basically the only difference between the 5d2 and 5d3 is the AF and thats worth $1500 in that case. but this AF UPGRADE isn't worth the same price? Im lost. and this is coming from a 5d3 owner.
no moire and 1.5-2 stops better SNR for video and a critical 2 extra fps and faster shutter response and no banding at very high iso are pretty big items too though, in addition to the AF (not a fan of the new VF though, easy replaceable screens would've meant more to me) oh and it's not super huge but it seems to do a solid 1/2+ better SNR
(granted it was sad they STILL crippled autoiso and they crippled video by not giving it crop modes, zebra stripes and focus peaking, ridiculous petty games by canon marketing and the dynamic range at lower ISO to actually become a trace worse when Nikon gets like 3+ stops better.....)
(and other nice ones, not that they really cost anything (other than perhaps the build): C1-C3 can now auto-update (really nice), dedicated video toggle switch, instant jump to exact 100% percent review, quick in field rating, audio meters, outline around histogram, nicer screen, less flimsy build, dual MFA for zooms (next they need to add near, mid and far distance settings perhaps), mostly they don't really add anything to Canon's cost and some could even be put on the 5D2 with a new firmware, but whatever, they do all make it more usable in the field absolutely for sure)