If the main reason for you to go to SONAR 8 is that it's said to be light on the PC, I doubt you'll benefit anything.
Every version new version of SONAR includes something the previous ones didn't, and the lightness is relative.
When S8 came out computers were more powerfull than two years earlier and so on. My guess is that 8 in fact requires more resources than 6, anyway. But as mentioned, I'm only guessing.
I'd say that if you could move to 8.5.3 you'd really get improvements. As far as I can recall there were quite a bit
of debate on version 8, as if it was a bit unstable/buggy on some areas.
I'm in the situation that no option for gear/SONAR upgrade is in sight for the next several years. I just have to hope my XP/SONAR 8.5.3 keeps ticking as well as it has for so many years already. It may well be that Bristol_Joneseys advice is the best - stick to what you've got untill you can get a proper gear/software upgrade.
And to clarify once more: As SONAR lisences are not transferrable ( and support for those old versions doesn't exist) you're totally on your own, on the illegal side, actually (but as mentioned earlier, who cares as so old versions are in question) You can't register the software, so you'll get no support, no updates nor upgrades.
I hope you'll find a satisfying way to continue with your music making!