I already have Dimension Pro 1.5 installed because of previous Sonar purchases. Is there any reason for me to reinstall it as part of the package that comes with X2?

If your current installation of Dim Pro is working well then NO. DONT't DO IT. Leave well enough alone.

Thanks. 
 

Installation instructions for X2 tell me that once X2 part 1 is launched the rest of the files will automatically launch until completion. If I do not download the DimPro files or the Loops and One Shots will it cause the installation to think it's not completed, or will it complete after installing X2 part 4 and then the Producer Extra content?

No it will happily finish. Dim Pro is a seperate installation. De-select it from the installation menu if given the option.

the loops and one shots are not the same as packaged with Previous versions.
they install to a flat folder structure but the new search facility in X2's Browser allows searching for all "bass" for example thus making how the individual files are stored irrelevant.

Thank you, sir!