I don't know what the optimum motor temperature would be, but a low motor temperature is obviously much better than a high one.
If the motor windings are not getting hot enough under high load for the controller to start reducing the power, then I would say that the existing cooling is doing a good job.
Unfortunately, I don't know what the default setting are for the VEC300 controller as I don't have one, but if regenerative braking is turned on in the controller, then you should feel a strong braking effect from the motor when the +5V brake wire is connected to GND.
If it is not turned on, I would expect it to just cut the power going to the motor without any motor braking effect.
Increasing the motor rpm by reprogramming would only be possible if the programmed maximum rpm was currently set lower than the motor's unloaded rpm @ 72V. According to the
5000W 72V dynamometer results, this should be ~4,260rpm.
The motor's maximum rpm will obviously reduce as load is applied to the motor.
Alan