I'm running this with a MOTU 2408mk2
There is a difference between the Mac Pro's CPU meter, which is in line with what you quoted, and the meter in Logic, which I assume shows the CoreAudio performance. Thus the assumption that we're looking at a CoreAudio issue.
There is nothing else happening in the Logic project. I have VE running on a dedicated second system and just for test purposes I set up a Logic project with just four software instrument tracks, each having a different VE instance assigned (again, audio coming from the dedicated second system via LAN.) Each instrument tracks is going to a separate AUX track where I have the convolution reverbs assigned.
So, overall, nothing in terms of system CPU intensive, or audio track intensive. Still, the Logic CPU meter will register an overload. So again, I think there's something else going on.