Thanks for the replies.
I've owned/used the Canon 300 2.8 with both extenders, as well as the 500 f4 with a 1.4x, and found the results to be unsatisfactory. It's fine for images on a screen, but not for printing anything larger than perhaps 8x10. I used both lenses on a tripod and a gimbal almost exclusively, so the 800 w/o IS on a gimbal is not an issue for me--if it's windy I usually go more mobile and use the 400 f5.6 and handhold everything. The 500 by itself is one of Canon's best lenses for sure, but I found it wanting on a few occasions for reach, which prompts me to look at the 800.
I've heard good things about the 300-800, but is a zoom range that wide really going to have better IQ than a prime? The other issue there is that the 300-800 is rarely available 2nd hand, while the 800 is much easier to find. The image sample thread looks like people are getting good results from the 300-800, but, regrettably, there is no thread (that I could find) for the Sigma 800.
Obviously, the Canon L lenses are going to be the gold standard, but the 3k price difference between a 4k Sigma 800mm and a 7k 600mm (version I) is enough to buy a lot of kit, and I haven't seen anything (yet) that would lead me to believe that the 600 is so much sharper that I should sacrifice 200mm and 3k for IS and a modicum of sharpness.
Anybody out there own/use one now?