The balls are Delrin, so would be the printed cages. There is no steel on aluminum.

Agree about the aluminum profile, there is false economy there.

I have cut till now at most 100mm thick. I cut at 200 mm/min and i don't push the hot wire in the block, rather the heat makes the cut, so no stretching or effort at that speed. I am not in a production environment, but even when i would be, what matters is the cut quality, not the speed. And it seems at that speed all details cut well.