I spent some more time with this today and got much better results. I forced myself to stay with 210C per FD_engineer's recommendation. Initially I had a "motor-clicking" jam (using my spring-loaded Drew feeder). I went to withdraw the filament, and it was jammed in reverse, too! So I took it apart and found that the filament had actually wrapped backwards around the feed gear. Once I withdrew it completely I saw some kinks that indicated maybe some other badness had happened while it was inside. Then I noticed my heater block was clamped such that the down tube wasn't butted right up against the feeder outlet; there was about 1/4" of gap (note this was not the case with my Afinia the other day, that one was
butted up against the feeder). So I moved the heater block up and tried again, and it worked for a while, then it stopped feeding. I think the cause this time was twofold: inadequate pressure from the spring, and too much pull required to get the filament off the spool. I tightened the spring a turn and babysat the spool during the print so there'd be no tension, and the print ended up coming out quite well as shown in the photos. Maybe not as good as FD's Makerbot print, but a lot better than my first try at 230-240C and 0.2mm.
I also tried "Normal" and "Fast" speed to test some of Roller's comments, but that was totally not happening, at least at 210C. The filament doesn't feed fast enough at this temp to keep up with the demand at those speeds, and the extruder ends up pushing a glob around instead of laying down a bead. Even on "Fine" speed it has some trouble keeping up when support is being printed. This might be related also to the length of the downtube - there's a good 15-20mm I think between the feed gear and the hot end, and you're basically pushing a very floppy noodle in there and hoping it doesn't buckle. I don't know if other extruders like the MB have a shorter distance there.
Next I want to see if these materials stick to each other and to ABS
210C, "Fine" speed, 0.25mm layer height. Form Futura Flex EcoPLA in black, NinjaFlex in red.