ER Collet Extension runout?

[mu38&Bg#]

Doe anyone use ER 11 collet extensions? I want to make a milling spindle using one and am wondering about the quality of the cheap import units. I'll be running up to maybe 20,000 RPM and using small tools, down to 0.5mm. I realize collet quality will be just as important. Before I spend 5 times as much for a name brand US made part, I'd like to see if anyone has any experience with the import parts.