Before I submit this as a real feature request, I wanted some input.

There is a lot of talk about setting buffer sizes for mixdown differently from buffer sizes for tracking (to change the CPU load/latency ratio). I've also noticed many people (most?) tend to freeze or bounce their softsynths when switching to the mixing phase. I was wondering if it might be worth/helpful to automate these actions and perhaps even make them a single button press on the control bar or something.

Off the top of my head, the way this would work there would be a mode switch in the control bar. Normally it would be in tracking mode favoring low latency. Switching modes would, at the very least, up the buffers to free up CPU for mixdown (more plugins). Additionally, perhaps it could also freeze or bounce all softsynths (freeze seems better since it could be undone by switching back to tracking mode). Other than that, the program would work the same way, as in you could mix while in tracking mode and track while in mixing mode. It would just be optimized settings to streamline things a bit.

If switching modes back and forth would be too problematic, the switch TO mixing mode could also be just a button that forces you to save the program with a new name (or automatically adds "mix" to a new save) and then bounces all softsynths and ups the buffers.

What do you guys think? Any unforeseem problems? Useless feature? Worthwhile? Better ways to do it?

Not a bad idea - maybe just high/low latency mode is enough, and these configurable in preferences.
Does all drivers have these buffers like ASIO - I'm not so sure of that - it may not be available always.
And some only through each drivers own panel - some firewire I tested did that.
 
But if any options to freeze etc, I would like that to be configurable.
It contradicts a bit the purpose - switching to high latency mode will let you run more synths in realtime - so less need for a freeze.
 
And mixing phase may also incorporate writing automation in realtime - and then you want short latency - to make it respond immediately on what you do.

 
Tracking is normally activated with the PDC button in control bar, letting armed tracks bypass pdc for monitoring.
Cubase got this ASIO Guard that has a similar purpose - latency is added unless rec-armed and configurated plugins say they are affected by ASIO Guard or not. You can set and forget kind of - nothing armed, higher latency.
 
 

Would not work in ASIO driver mode, there is no way for SONAR to instruct the audio interface driver to change buffer size.

Good points. And a shame, too!