Is there a built-in shortcut or some other quick way (X3) to switch between Mix - FX - I/O in the track view, instead of the dropdown menu at the top of the track pane?

I haven't started using AutoHotKey yet, but scook can probably confirm if that will do what you want.

Yeah, I don't think it can be done without a 3rd party tool like AutoHotkey. I am not very adept with AHK but managed to get a working script. I am sure a Windows developer could supply a more elegant solution. Here are the first three options from the Track Control Manager drop down bound to Windows-1 through 3.
#1::
  SetTitleMatchMode RegEx
  ControlGetPos,x,y,,,AfxFrameOrView110u12,SONAR.*
  x += 300
  y += 15
  MouseClick,Left,%x%,%y%
  Send,{Down}{Enter}
  Return
#2::
  SetTitleMatchMode RegEx
  ControlGetPos,x,y,,,AfxFrameOrView110u12,SONAR.*
  x += 300
  y += 15
  MouseClick,Left,%x%,%y%
  Send,{Down}{Down}{Enter}
  Return
#3::
  SetTitleMatchMode RegEx
  ControlGetPos,x,y,,,AfxFrameOrView110u12, SONAR.*
  x += 300
  y += 15
  MouseClick,Left,%x%,%y%
  Send,{Down}{Down}{Down}{Enter}
  Return


 

Yikes! I hope you didn't have to write that code. If that's the only solution I'll stick to pointing and clicking. I find myself doing that a lot lately, so thought I'd ask if anyone might have found a fast method. Thanks, Steve!

Well I did not HAVE to write it but the code did not write itself and I did learn a little more AHK.

scook
Well I did not HAVE to write it but the code did not write itself and I did learn a little more AHK.

I haven't ever seen AutoHotKey, so I don't know how it works. What I meant was I hope the program generates code automatically in the background in response to actions the user does in the foreground, kind of like a macro recorder. Since that's apparently not the case with AHK, thanks again for your effort and when I have some much more complicated repetitive tasks to do in the future I'll give it a whirl myself.

AutoHotkey is a pretty handy scripting tool. There are keystroke capture routines but I don't use them. In this case I am not sure how useful they would be. The tricky part (for me) was trying to fix the location of the Track Control Manager drop down. That is where
ControlGetPos,x,y,,,AfxFrameOrView110u12, SONAR.*
comes in even though it is not portable between SONAR versions. For example, in X2 the object is named AfxFrameOrView110u13. It still looked liked the best bet for invoking the drop down. But as I said before, this is new to me too. Anyway, it was a good exercise.

There's actually a default keyboard shortcut for this. I'm not at the daw right now so I can't tell you exactly what it is, but it's one of the modifier keys plus the left/right arrow key.

You the man, Tak! It's SHIFT + L/R Arrow. My life is better already. Thank you!

Wow, that's great to know  
 
And while we're on the subject of "incredibly useful but not obvious keyboard shortcuts," don't forget that Shift+Ctrl+L/R Arrow tabs through the multidock.