Upload & Sell: On
Well, after testing the Tamron out with my 7D today from my office window which overlooks an interstate, I can additionally confirm the issue exists. 6D seems ok. However...as was suggested above, taking your finger off the shutter button and basically letting the lens "catch it's breath" for a second seems to wake it up to the fact that it's gotta track something new.
On the positive, it retained AF on moving objects over 10-12 frames no problem. So for me and my uses, I don't see this issue stopping me from enjoying the lens (racetracks etc.) but if I were a birder I'd consider it a bigger issue.
I guess the reality is that at $1k and 150-600mm zoom range, something had to give. The IQ seems to be very good, build good, and good VC, so an AF system that seems to struggle with certain Canon bodies under a demanding AI Servo usage appears to be the "give".
For me, it isn't a big deal, but for others I could definitely see this being a deal breaker.