I haven't downloaded 6.5 yet, but will do so over the weekend and report back.
I only upgraded to an x64 OS a few weeks ago, so my experience with 6.4 is limited, but thus far I've had no problems. When I 'm getting close to 100% utilization on my two UAD-2/Solo cards I tend to back off, perhaps I'll have to try pushing a bit harder?
As far as the rumor about Sonar support goes, I received an email from UA a while back assuring me that they do support & test Sonar, and at least for now there is no plan to change that. I've asked if I can repost that email, but haven't heard back yet.