Sonar 2.2 Really

Maybe. SONAR's pretty good about backward compatibility, but there was a major change to the project file format in either S3 or S4. You will also likely have issues with missing plugins. So "maybe" is as good a guess as I've got.

You might want to consider just keeping 2.2 on the old machine for the old projects and start afresh with only new projects on the X2/W8 machine.

One thing is your audio tracks will still be easily transported over. 
Not sure about MIDI as those are embedded in the CWP file extension. But what could be done is just export the midi tracks as MIDI and re import to a new project in X2. 
As Bit say's it will be a matter of plug ins but the new ones are most likely much better anyways so no loss.