but my guess is that it can only shift processes to different processors, but not threads, as a process can't run (efficiently) on two different processors.

Why do you say that?

To quote from this AIX Programming Guide:

On a uniprocessor system, threads execute one after another in a time-sliced manner. This contrasts with a multiprocessor system, where several threads execute at the same time, one on each available processor. Overall performance is improved by running different process threads on different processors. However, an individual program cannot take advantage of multiprocessing, unless it has multiple threads.

Indeed, I am not aware of any flavour of *nix that supports POSIX threads that does not allow threads to run on all available cpu's? Nor am I aware of any efficiency problems inherent in doing so.

Is this just FUD or do you know something I don't?