Do you have any type of dampening on the steppers? harmonic dampeners? If not then I suspect that your motors are growling something fierce when you are at speed.
Do a search on the zone for harmonic dampeners if you are not using them already. They make a world of difference with your setup.
No aluminum, yet. Just Delrin and Wax. The Bosch spindle is great on those materials.
And thanks for the tip on harmonic dampeners. I'll have to look at that.