Since making sure every one of my 15 Event Input plugins had a bypassed IO utility plugin on it, I have had no MIDI dropouts or silence on playback/record issues.
I am running one instance per slave machine and this is working really great... Practically no CPU usage on either slave Mac Mini. As discussed elsewhere one CPU core on my host Mac Pro has lots of activity, but so far no dropouts, clicks or overload messages due to this despite running a massive 256 track orchestral template.
So the software functions exactly as it should. The single core CPU mega-activity problem needs to be solved and be able to be spread over more cores. This sounds like a Logic issue when lots of tracks are forced into live mode with the IO plugin - still Vienna can surely fix this as you are able to run loads of Kontakts in Logic all in an apparent live mode without 1 core going bananas.
Best,
chris.