Improvements in real-time playback engine efficiency are one thing, but the sound of an offline rendering should not have changed if the project didn't change, and the same export settings were used. And 8.5 already had the 64-bit double-precision mix engine. But you'd have to compare the raw master exported from 8.5 to a new master exported from Platinum without out any other Wave/CD mastering app getting into the mix.
 
Yes, the new interface will sound different on playback, but it won't affect export, and the old master played back on the new interface should sound the same as a new master.
 
If you still have 8.5 installed, you can check this by creating new masters from each. They may not null if there are instrument or FX plugins involved that have random elements, but you can set up a blind A/B test, and see if you can reliably tell the difference.
 
I haven't tried this for a while, but in that past when this subject has come up, I've not been able to detect an audible difference between exports from different versions done back to back.

I did this last night, Eport the same project from 8.5 then Professional - you are correct , I hear no difference at all. So I guess the issue is possibly with the Loudness Maximizer. I've never noticed that it trashed the sound but I guess all limiters are guilty of some sort of sonic change to the mix. 
The good news is I've already made a couple of way better mixes of these songs. I found I had the vocals down to far. And Pro channel is the best thing that ever happened to Sonar. 

Maximizer.... ;-/   I would have spoke up earlier, but I just now read this thread.  People rave about L2, and it's competition....  Just hearing more dynamics can do more to your brain than, what you think it can.

The sounds are the same with all the EXACT SAME settings. You are experiencing the placebo effect. If you think something will sound better, then it will to you.
 
CJ

Do you know where I can get a free 64-bit placebo plug-in?

with Melodyne?

To re cap.
I fired up some projects from 2010 ( Sonar 8.5 / 64 bit)  in Professional   ( 2015) 
I immediately "perceived" things were sounding "better" 
So I imported the Master Wave file for the same song into the project and there was a difference for sure. I optimistically assumed it was because Sonar's audio engine has improved or something along those lines. Therefore the title of the thread. I was wondering out loud if anyone else might have notice an improvement. 
Thanks to the collective mind here I did some tests and have concluded that there is no perceivable difference between using 8.5 and Professional. That is when using my present system. 
I don't think the system can be involve with an exported file but??. 
The unknown to me is how much an audio interfaces drivers can the screw the export up? I have solid proof that the old Tascam drivers that I was using at the time trashed my timing offset. But all overdubs where done with the Tascam so I doubt if my Focusrite drivers would magically re align that. 
 
The last test I did to qualm my thirst for understanding was to re master the song in Wave Lab and I applied 1.5 db of gain with the Loudness Maximizer.  
When I compare the old and new master there is a noticeable difference in sonic clarity ( for lack of a better term) Both are same RMS average of 14db. Like I say, it's like the old master might as well be a 64bit MP3 compared to the new one.  It's like pulling cotton out of your ears. 
So if it was the fault of Wave Lab 5  (32 Bit??) then it could be my upgraded version of Wave lab is now responsible for the improvement. 
 I guess I'll leave it at that and just re master the album with my new improved system. 
 
 
 
 
 
 
 
 

It sounds better, it is better.

I wonder how many tens of thousands of CDs were mastered with Wavelab 5 and now have to be completely redone?  Eeek!  :P

konradh
Do you know where I can get a free 64-bit placebo plug-in?

 
 http://www.bozdigitallabs.com/product/rack-spacer/
 
 
This plug has made my mixes sound 20% - 40% better.