So the EERO 6 experiment did not work. Why? Because no matter what I did, EERO 6 wouldn't connect to Sonic Fiber and I couldn't set it up.
But the good news is that on my old EERO 5, after I have restarted it, the speed test came back as 390 down and 593 up, which is closer to what I was getting on Intel's Air. The Tx Rate seems to be holding at 866 and the speed test on the EERO 5 itself is 950 up/down, so the EERO 5 is running at the fiber's speed...
Maybe I'll try to set up EERO 6 again. I'll update this if anything changes.