I actually get fewer xruns in callback
        mode, 
      This sounds highly unlikely. Maybe your "delay" setting is too
      low? Or Pd is not actually running with realtime priority?
 
 
I spent a lot of time testing this on an ancient laptop running a fairly modern kernel because I was deeply frustrated with general PD performance on that machine. "unlikely" it may have been. True, it most certainly was.
    
    
      
I also bump the sound-generation process
        up to realtime priority.
      Pd itself already tries to raise the thread priority; if this
      fails, you might not have sufficient permissions.
 
 Perhaps. But I was definitely able to manually bump the priority without sudo. I did a *lot* of system optimization trying to squeeze good performance out of various moderately complicated physical models. From this 
I learned a few things:
1 - the PD UI will absolutely kill performance
2 - because of 1, plan to run the sound gen in [pd~]
3 - pd patch and file lookup is a nightmare, especially in [pd~]
4  - real-time priority + callbacks usually produces clean sound on a clean system
And now you know why I want multi-threaded pd audio.
- d