Please forgive me if you have posted to this kind of question before, I am new.
11-2011 MacBook Pro 17 inch (I7 core), 500Gig Solid State Drive, 16 Gig Ram. OX 10.8 mountain Lion... Native Instruments Komplete 8; Analog Factory. Scarlet 8I6 card... (I know)
I have been running Ableton Live 8 as a layering host. Meaning I play live, and essentially have about 6 tracks up, and monitored, and I play them all live as one layer of six voices. I just adjust volume as I need "voices" inside songs... I use piano, a couple of electric pianos, some strings and pads, etc...
got to thinking it might be overkill for what I used it for and frankly didn't like all the sounds coming with the live 8 suite.
In ableton I was getting 35% usage on my CPU. Sometimes 40 to 45%... Then I purchased Mainstage and basically slammed my CPU. No changes. just set up layers. And only four of them! I tried a couple of 'blogged' solutions like shutting down wifi.. lowering volume controls below 0.0 db. but really, I don't understand why I have to kill applications, and shut down all kind of things to run it. Please continue to troubleshoot it if you have not.
On a side note... if this doesn't work out, can you recommend a host that can play layers of AU's? and has a low profile? Something like Mainstage can do? it seems it is really set for live layering... but maybe just didn't quite get there?
In my experience Logic can consume way more CPU running Aalto than other hosts. I'm guessing this is true of Mainstage too, though I haven't tried it much. In general, hosts differ a lot in CPU use. You can try some others and compare.
I don't recommend Apple AU Lab a lot because it's not really designed for musicians... but for just running AU plugs like you want, if your setups are not complex, it might be great. It's free, give it a try: http://www.apple.com/itunes/mastered-for-itunes/
I would also give Numerology from Five12 a try.