Win 7 vs 8.1

Sonar runs smashingly well on 8, 8.1 and 10.
 
Windows 8.1 has some pretty significant changes to thread scheduling and memory management and sports greatly improved multi-core performance compared to Windows 7, and XP just isn't in the same league. To me the only valid reason to stay with Windows 7 is if you like the Aero desktop. But that's just me; I'm an inveterate upgrader with mommy issues.
 
To round out the story, Windows 10 further improves these functions and also has code to improve DPC latency issues by pushing stuff like network and video interrupts to other CPU cores, letting audio have CPU0 for its interrupts. The team's stated goal is to make DPC latency issues for audio apps just a bad old memory. I'm not sure how far along that path they've gone yet, but I believe there is some of that work in the current release.
 
There are also major improvements to the audio stack itself, making WDM performance pretty equivalent to ASIO. If you use ASIO, you won't see much change, except for perhaps being able to run with a somewhat lower sample setting. There are new application level opt-in features in the audio stack for even better performance. It will be interesting to hear Cakewalk's plans for leveraging those features.
 
Anyway, it's an exciting time for audio on Windows, whether you go 8.1 or 10, and Cakewalk has a great history of making full use of the improvements MS builds in to each new release.
 
Your mileage may vary, best of luck whatever you do! 
 
Edit: Oops, just saw that you decided to move up to 8.1, good choice!