denverdrummer
Isn't the only difference in content (other than new content with X3) is that X3 doesn't have R-mix? If so you should be able to uninstall X2, do a custom install and only select R-mix. Just don't accept the default install and you can go in and check off what content you want loaded.
More than R-Mix as dropped from X3. V-Vocal was dropped and does not have an advanced install option. In order to install V-Vocal, the SONAR X2 program must be installed. The VC-64 was also dropped and can be reinstalled with the advanced install option. There may be other plug-ins dropped but these are the only ones I am sure are not in X3.
With the exception of the demo/tutorial projects, removal of the parts of X2 Producer that are not shared with X3 would recover about 200MB of disk space.