Bohns, the bokeh with extenders isn't going to be as pretty as a raw lens, but it depends a lot on the subject in the out of focus area and how far it is from your subject. Generally complex textures like small leaves and thick grasses can look odd if they are right next to your subject.
I find that f/8 is noticeably sharper with this combination, but also used Focal to perform AFMA. I don't have a ton of room in my house, so I did the calibration at roughly 30ft./10m, and obtained good results. I'm pretty sure I've read in Focal or Canon's literature that the 50x distance is more important for shorter focal lengths and as long as you were reasonably past MFD you can obtain good AFMA. I have found that my results often better at around 20-30x, versus 50x due to the target size, as you say.
I used a solid tripod/head weighed down with 20lbs of sandbags, wireless remote, very well-lit target (+11EV) and did a Manual calibration at -20,-15--10,-5,0,5,10,15,20 and after running the test, re-ran it at +5 through +9 to get an even more precise calibration (of +8) from the +7 it recommended during the first test.