My motor is NEMA17 - 42BYGHW811 - 4800g-cm /// 2.6A /// 1.8 deg/step.
The first test was to push the filament. It was pushing it with no effort. The motor and the stepper driver was cold due to process. I just make it pass thru the extruder. I've made some test with the bearing close or far from the MK7 drive gear and I think I found the place that it's most efficient. I tried to measure the pushing force and it's between 200 and 700 gr (pretty rough but...electronic scale for the win!). Only the spring must be stiff. I'm using a spring from IKEA clothes pins (they are pretty stiff).
Notice that I still didn't test it with a hotend. I am building a Delta printer (more than 200 cm tall - 500x500x1000 building volume) for a friend and I plan to use this extruder for it. When his hotend come, I'll test everything with it.
On my standart Prusa i3 I already put your extruder! Nice project and I'm really happy with the results. I upgraded my printer with 1.75mm filament.
Best regards and keep up the good work!