Since the PWM controller isn't 100% efficient, another means of obtaining yet more speed (if you frequenly drive with your foot on the floor) would be a controller bypass. Based on controller efficiency figures I've seen tossed around on the EVDL, I think you'd see at least another 10% speed increase. Plus you could still use field weakening on top of a bypass circuit.
The challenge would be setting it up in such a way that didn't damage the controller from back EMF on switching the controller back into the circuit.
Thinking more about this Tom - I think I can see a field weakening experiment in my future. Not predicting when, but it's a far easier thing to set up & manage than a controller bypass.
One of the EVDL Yodas has outlined possible hazards of using field weakening in concert with a PWM controller (potential for damage), and recommended several safeguards: