Update:

Extrusion is still the fight. About half way through one of my latest prints the stock stepper motor basically just gave out and started popping/clicking 100% of the time with very little plastic extruding. I couldn't get the binder clip positioned in any way that improved the situation.

The next day I started researching replacement Kysan stepper motors and found they have a distribution facility 15 minutes from my work. Sam, the rep that ran the warehouse (which is basically a lounge with some shelves), hooked me up with a drop in replacement stepper (1124144) that outputs ~20% more torque than the stock stepper (1124090). I also got a 5:1 that is like 10x more torque, but it has a larger shaft and I don't plan on trying that until I max out the drop in replacement. The 1124144 seems to work great other than some popping on the first layer (which uses 150% extrusion width). I am going to experimenting with smaller extrusion widths to see if it helps the first layer popping. I also need to pursue a better extruder design. I'm still using the stock one. I started printing a "Shanes Extruder", which is a popular design on the Blue Eagle Lab forums. I think with the new motor, extruder design and a MK8 extruder gear, I should be able to really start increasing my speeds.

Lastly...I'm experiencing some fairly bad excess extrusion during retraction moments ("ooze" is the accepted technical term). Basically, the hotend retracts and the extruder stepper stops, but ABS keeps extruding. I'm thinking this could be a temperature or extruder calibration issue. Not sure yet. Either way it's a problem and my hotend ends up hitting the excess ABS globs that get deposited, which in turn causes it to shift a few millimeters to create a "step" in my print layers. Subsequent layers return to normal, but it happens fairly often if I don't remove the globs. I just found this, http://manual.slic3r.org/expert-mode/fighting-ooze, and have a lot to try!