Paul,
I side with noiseboyuk's assessment. I also have issues with Cubase Pro 8 running on a 12-core Mac Pro (2013) Mac OS X 10.9 with plenty of CPU power and am getting severe CPU spikes, audio dropouts, and distorted audio across the complete audio buss as long as ASIO-Guard is enabled.
In my template I have 1,000+ MIDI tracks connecting to 50+ VE Pro Rack Instruments that connect to three workstations. I have not tried to run with the ASIO-Guard lowest setting, as the other two settings caused such severe issues that I simply just disabled it.
Just another viewpoint. :-)
Jurgen