The mixtreme drivers are first class. They run very efficiently at minimum latency, have true multi client support -I think its called - where multiple apps can access each asio stream simultaneously.
It allows integration of all running apps into one infinitly routable mixer.
Rock solid performance you can depend on.

Well you know that, but I confirm asio will not work over multiple cards.

You need this

http://www.sydec.be/Products/Details/?ID=a97e9af3-cc4f-46f0-a24e-f6b7f97d2933

I am getting one and will connect to my 64 a/d's via MADI
A complete solution for all routing, monitoring, multiple apps, external effects, cue mix's, including eqs compression and fx. As long as you have some hardware control.