I'm looking to see if there are any existing ACT XML import files for the Yamaha MX series synths.
 
I found plenty of threads for creating the .INS files for the MIDI Instrument definitions for the MX keyboards, but nothing about the ACT XML files to import the control surface information.
 
The MX series have a mode for controlling the DAW "remotely" and even have a setting to be SONAR specific. All fine and dandy, but I haven't been able to find any of the ACT XML files that the "HELP" function suggest ought to exist. I imagine I can go through the ACT Learning tool and setup up SONAR to understand the MX - but I was hoping it was already done and available.

I checked the knowledge base, but I didn't find any pre-existing XML files to import. I've managed this long without using my MIDI keyboard to control my virtual instruments, but since the MX has knobs to tweak the VSTi and transport controls as well, it'd be nice to have the option to use them.

You mix things a bit...
1) MIDI controller can "speak" directly with VSTi, the same way as keys trigger notes. And as with keys, DAW has no influence on that process, you will need to "MIDI learn" inside particular synth (if it support learning) or configure MX to send what synth except. That is the simplest, quickest and strait-forward option, especially if you want to tweak just 1-2 favorite synths. But sure, there is other option...
2*) I do not recommend to use Sonar "Remote control" is your case. While that is second in simplicity, controls used still "leak" into VSTi. The result (synth dependent, particular CCs on your knobs) can be very frustrating (in addition to the modification you was expecting from "remote", you can modify some parameter withing the synth. Sometimes hard to find which one, or not notice the change for a while)
3) for transport in any case and for VST(i) control if you want, you need some "Control Surface plug-in". It will intercept your operation of buttons/knobs  (they will no longer "leak") and do something in the DAW (change volumes, FX/Synth parameters, etc.). Here you have a choice:
* Generic surface. Probably the best to start with. You can learn transport and several knobs for ACT (Dynamic plug-in mapping) mode. You just need to "ACT Learn" particular plug-ins. Does not take long. But:
- read Sonar reference guide (2000+ pages one, not introduction), dedicated section for Control Surfaces. It is very hard to do everything correctly without that
- do not mix "ACT Learning" with "Remote control" nor "MIDI Learn in VSTi" nor "MIDI Learn in Surface plug-in". While technically you map a knob to something, this "something" is different in all 4 cases.
- find ACTFix utility on my site (see signature) and use it. One unlucky day all your mapping will be corrupted otherwise and you will have to start from scratch (till you manually backup files).
* ACT MIDI. A bit more complicated, with a bit more possibilities.
* AZ Controller. Ultimate power, the price is the complexity... But you can try "Startup preset" (there is YouTube video), which will bring you up to "ACT MIDI" level quick (the difference is that with "ACT MIDI" you can not do more, while with AZ Controller you can).

I forgot to add.
Only ACT Dynamic Mapping is saved in XML. Everything else has binary form (presets can be exported as SPP).

That's good information. The remote daw is the setting of the MX49 that tells it to send the transport controls and remaps several of the voice selection buttons to control the DAW. I haven't actually tried the knobs on the keyboard with a VSTi. Up until now, I'd be purely mousing it.

I'll have to see if the ADSR and other knobs send midi information when not "remoted" - they do when set to DAW mode.

I tried to find Control Surface plug-ins for the MX - and most results landed on pages telling me to find the ACT XML file - so that was a cold trail. It probably doesn't have any. It was designed to control CuBase (Yamaha and Steinberg work well together for obvious reasons) - but does have a setting for SONAR. And the menu direction controls do let me navigate around the tracks and track parameters. But that's all the out of the box SONAR setting works for - everything else will have to be learned. I did use the Cakewalk Generic Controller to attach the MX when checking it out.

I repeat, ACT XML files are just for mapping "virtual" controls to controls inside VST. Once everything is fine, it takes 1-2min per plug-in (one time, the mapping is global and not project dependent) to create these "ACT XML" from scratch. And you can map physical knobs to "virtual" controls f.e. in Generic Surface ACT section.
 
But there is no dedicated full features MX ACT plug-in (binary code, with binary settings), as they have for Cubase. With Generic Surface you can quickly map some functions (one control -> one function), but thats it.
What you can read in the MX reference guide about it and Cubase is implemented in special software. You can achieve (close to) the same "deepness" with Sonar only by writing your own C++ program or using AZ Controller. Both approach will take significant effort and time, the second is simpler and require no knowledge in C++.

Well, I re-read the MX reference manual. First, there is an "MX Remote Tools" that may have added the binary bits I needed when I installed it. It gives users control on the knob CC messages, you can fine tune it for a specific VSTi, I believe. Also, I missed a step in the original read through of the reference manual. The MX uses the "Mackie Control" set when put in SONAR mode, when I added the control surface to SONAR, I needed to specify "Mackie Control" rather than "Cakewalk Generic Surface." I was able to get the transport functions, at least the default ones, like play and stop, to work. 
 
The MX has a bunch of VSTi it understands out of the box, apparently, but I don't have any of those. I've only used Rapture or Dimension instruments so far - which perhaps not surprisingly are not in the Steinberg repertoire.

Two things are missing in the SONAR settings compared to CuBase - The data wheel, aka the AI Knob, is mapped if you have CuBase so you can put the mouse over a setting and can spin the wheel to adjust that setting - not a big loss but it would be handy, and CuBase can apparently use the MX as a VSTi somehow - not sure how that helps, though, if I'm actually playing on it directly.

Now I just need to set the other control buttons to have functions, like "start record"... that would be nice to be able to control from the keyboard - and maybe comping and file save, and I'll have to think of what all would be good to not have to reach for the mouse to do.

SandlinJohn
Two things are missing in the SONAR settings compared to CuBase - The data wheel, aka the AI Knob, is mapped if you have CuBase so you can put the mouse over a setting and can spin the wheel to adjust that setting - not a big loss but it would be handy,

Sonar does not support that in general. The closest you can achive is continue controlling a parameter in FX which you started to modify with mouse.
But I think you can map AI Knob as permanent jogger in Generic surface and probably it already is the jogger in Mackie mode...