So, I've gone nuts, and installed a 0.9 degree stepper on my extruder. I'm running Marlin 188.8.131.52 on the Predator, and had previously calibrated the extruder to 410 steps per mm. So I upped that to 820 to compensate for the new motor. I had to swap two wires, and the results are well worth the effort, even without fine tuning or calibration. No ghosting, no salmon skin, no blobs, no stringing.
Tomorrow, I'm going whole hog and replacing the axis steppers with 0.9 degree units, and replacing the drive pulleys with genuine Gates 16 tooth units, the belts with genuine Gates 2GT3, and the idlers with genuine Gates 20 tooth units (to replace the stock smooth idlers). I've also got an E3D V6 hotend, which will take a bit of machining to install. But steppers, belts, and pulleys first. Just placing the new Gates belt in the new Gates pulleys feels distinctly more "precise" by hand, so I'm looking forward to the results.
I'm also going to install Sorbothane 40 OO pads under the feet to help in killing mechanical crosstalk between the x,y, and z motions. With these mods, the machine is down to 5 micron resolution.
I obtained all of the parts (motors, pulleys, belts, hot end) from E3D-Online, and they all arrived 3 days after I placed the order. At least for the extruder motor, I didn't have to do anything other than re-set the steps per millimeter, and shorten three screws on the extruder because I'm direct drive (no bowden).
I'll follow up after the axis motor, pulley, and belt transplant.