It’s just a suggestion. But you do have a point about the HDD/performance meter not maxing out… Anyway all is fine here even when using built in audio meter is always below 50% with upto 7 FX Channels and 3-4 group Channels. All my plugins and gear are listed in my sig all upto date. Good luck I hope you all resolve this issue…
I think the discussion of using multiple drives and optimizing drives is still valid when considering ways to enhance system performance, especially given the very puzzling nature of sync24’s issue.
I’ll be very curious to hear what SB tech support has to offer.
Cheers
In addition to this, power management on the MBP was also an issue for me. With an external monitor, I needed to run in clamshell mode with the laptop open which cuts power to the internal screen resulting in significantly reduced fan activity and less CPU strain. If your interface is bus-powered, this might be a contributing factor as well.
This problem has been FIXED 
In my quest to track down this major problem that was rendering my DAW pretty much useless, I managed to get a copy of Logic Pro x 10.0.6 with a view to reviewing it’s suitablity for me and with the plan being that I was going to dump Cubase entirely and move to Logic.
Whilst planning this move I did extensive research on the net. Pretty much everywhere I went I read that most people where slating Cubase and it’s performance on a MAC saying it was ‘no good’. So I thought It can’t do no harm to look at alternatives.
So I loaded up Logic, got it set up and went about stress testing and within a few minutes would you believe Logic was doing the same thing, coughing stutterting, poping and clicking. 
I was so p!$$ed off - I really didn’t have a clue on what way to turn short of buying a new MAC.
Then, last Monday - my OCZ Vertex SSD gave up the ghost and lost 2/3rds of it’s capacity - I managed to get the laptop into recovery mode but diskutil was showing the 512G drive as 132.48G in size
So I ordered a new Samsung 512G EVO, installed it and recovered the system today from my time machine image.
Tonight I thought I’d go and do some more comparisons between Cubase & Logic to see if I could get anymore info as to why my system was struggling.
I loaded up my ‘stress’ project and hit play in Cubase and to my amazement - playback was flawless - no glitches, clicks or pops AT ALL. I was able to lower my buffer size down to 256 and add more ‘stress’ tracks without a single problem.
So, what do I think went wrong? In my opinion, I think the SSD was slowly dying without my knowledge anyway and that the OSX Kernel was struggling to write data into virutal memory {swap} or perhaps index the drive or carry out any other low level kernel IO which is why I saw kernel_task often working hard - this is turn was upseting CoreAudio and Cubase 
My DAW is back, rock solid and coping with pretty much everything I am throwing at it at the moment. As for me? Well I am a happy bloke again and I can now try and get back into the groove!
Hopefully this might help someone, someday!
Cheers - Kev.
I’m happy for you