I question the objective of building an inaudible HTPC. To begin with the computer is generally located close to the monitor/TV and a meter or two away from where one is sitting. The other thing is that aside from near silent parts of a movie or a piece of music, the audio of the material will swamp...

I would assume that the voltage/current/RPM specs are for the fans working in free air. For another data point I looked up the current draw for the S-Flex E: 0.15A @ 1,200 RPM. Below is a small chart comparing the current, CFM, power used and efficiency for the three fans (based on published specs.)...

It has nothing to do with the mechanical aspects of the fans (bearings). It due to the differences in the motors. As you noted, the Slipstream has a lower staring voltage, and probably draws more current at 12v. Are you sure that the fan header isn't under BIOS control and is delivering a full 12v? ...