Thank you for your feedback to my questions.
I believe that it depends on the power vs speed graph for the motor, for example I went to the HSD website and pulled the spec sheet for two spindles at random:
http://www.hsdusa.com/bo/allegati/Fi...fnz4100919.pdf
http://www.hsdusa.com/bo/allegati/Fi...4100917_1_.pdf
You can see that the rated current, which is presented in table form at the bottom doesn't change much over the speed range, it's certainly not proportional to the RPM. Of course these spindles don't max out on power at max RPM and are not a straight angled line to max power at max RPM as some of the less expensive spindles are. So I'm just saying that it depends on what spindle you have.
Yes, I've seen a few monster spindles with ATC for cheap as well. The thing you have to be careful of is some of those spindles have specific requirements, like lubrication systems, where even a new lube cartridge will cost more than you got the spindle for at surplus.