Upload & Sell: Off
I did a few quick comparisons with my Kenko MC and Canon 1.4X II and the Kenko was at least as good in the center (slightly better if anything) but a touch softer in the corners (which I think is opposite of what is discussed in the link). At any rate, the differences were minor IMO and I would say they were comparable (I had to view images at 1:1 on my monitor to see differences).
I would expect the newest Canon TCs to be better...probably noticeably so in the corners...but I have no experience with them and for 3X the cost I probably won't bother since I'm mostly reach-limited and cropping out of the center when I use the Kenko (i.e. not really concerned with corner performance).