I am working in Cubase. At this moment, I have the buffer at 768, I'm mixing. I feel like I could add convolution til the cows come home. I have 16 instances up of Vienna Suite (two of them are cathedral convolutions, of 3 convo verbs in all), six Kontakt multis (all loaded with all sorts of convo inside), BFD2 exporting 18 audio tracks on top of the master stereo (a 32 piece kit, almost all being used), and about half a dozen other audio tracks. And some Absynth, two instances. A couple amp sims in Guitar Rig3. (I did finally freeze two Kontakts). (The Convo here isn't really making that much of the dent...)
I'm hitting the CPU meter at 16, 17% in this project. The cores are fairly, though not absolutely evenly working, by the graphic crawl in activity monitor. It's fricken Godzilla here. I don't think you could blame core audio. I am using four drives, though, and they're *managed*.
What audio interface are you using?