Based off of your datasheets, your Z / extruder motors in particular have an extremely high inductance and would probably benefit from a much higher supply voltage. (Higher than the TMC2660's support) I wish that more (Or any, even) of the single IC stepper motor drivers supported 48V.
If you have continue to have speed issues with the Z / extruder motors at 24V, I'd suggest going with a lower inductance motors for these two - Something like the 34mm one from here:
http://www.ebay.co.uk/itm/122293784943
When picking motors, you essentially want the one with the highest phase current for the torque you require. (Within the limits of the drivers you're using, of course)