Adjusting the tension on two rollers is a tricky problem. Its partly why my design has so many gears. That Robotdigg drive gear looks interesting, I havent seen those before. As you go faster you will always find more limits, whether its drive gear grip, motor torque, heater power, or hot end flow rate. You just have to work out whats limiting you and fix it =) Though I would say once youve hit the max flow rate of your hot end thats probably a sensible place to stop. What extrusion volumetric flow rate can you achieve with your setup (in mm^3/s)?