I've been looking over my shots from yesterday. I just used the 300 w/no tc. What I noticed last night on a couple of shots of gulls, was that even on iso 100, I was overexposing the gulls on the sunny day...leaving a transparent white (not chromatic) halo around their heads in high contrast situations (with the tc's this is the area has CA). I'm thinking that where my 5D and the 30D tended to underexpose everything, that the MKIII may be tending towards overexposure...I automatically use + exposure comp because of the way I always used to shoot...yesterday I dialed it down to 0 (tried both evaluative and spot metering) and still came up with overexposed heads. When I adjusted the exposure in lightroom, for the heads, the bodies were under exposed but the halos went away.
So, I'm thinking my "ca" issues must be exposure related...