I've been using Sonar 8.5.3 since 2010, but just got a free copy of X2 Studio (for purchasing an Octa-Capture)
 
Is it worth my efforts to install X2 Studio?  I'm concerned about whether all the "extras" that I got with 8.5 Producer can be installed into X2 Studio without any licensing issues, etc...
 
If X2 isn't THAT much of an improvement over 8.5 (again...remember I'd be going from the Producer version to the Studio version of X2)...would I be better off not using the new version?
 
 

As far as I know Sonar software has always allowed all versions to be installed on the same computer without any issues or conflict. What you'll get is the extra plug ins that come with each version. 

I have Sonar 7 32 bit- and 8.5/ 32 and 64 all on the same machine. I also have  the demo of x1 which expired and I cannot run but it's all there. Each had a different serial number and activation code.
 
Having Studio will give you a chance to see if you like the new interface. If you look at the comparison charts you might find your not missing much anyways. Your 8.5 64 bit plug ins will be available ( I think)