Umm, probably just bearings. I would have used bearings for the back end of the motor shaft but I wanted to weld it on and I didn't know how well bearings would hold their integrity when subjected to extreme heat.

The collets don't really inhibit spinning much. There's marginally more friction because the shaft is spinning in something that's not moving or has rollers or ball bearings, but it's not particularly noticeable and I'd be surprised if you saw a significant reduction in rpm as a result of using a collet. I was planning on using these 8mm collets as a form of bearing for Paws' spinning disc (although I'll have to buy some more now!)

The only issue I can think of is that to limit movement along the shaft, these collets need to be fixed in place with the supplied grub screw, which then obviously prevents the shaft from rotating in the collet. This can be avoided depending on the application though, so it would depend on how you plan on using them (or mounting them) really.