What I'm doing now... which seems to be closest I've found to what I want to do, is this...
chop audio stems into 4 or 8 bar clips.
Enable groove looping. (Sonar gives its own idea of the original audio bpm (seems quite accurate)
Un-enable groove looping. (*)
Enable stretching. (*Sonar sets the clips tempo to project tempo)
Type in the bpm it gave me previously in step 2.
Audio's tempo appears to be roughly adjusted 'on time' to project.
Some bits fall a little off, whether that's down to the musicians or Sonar. But it seems to be getting close enough.
If there's an easier way I'd love to hear.