Hi there!
I'm very interested in the sonnet expansion chassis (and magma 3 port chassis) as well... I have one of the sonnet on order already, and keenly awaiting their release.
I'm just finishing setting up a completely portable composing rig... 17" macbook pro, 2xmac mini's (and at least one more on its way) all communicating via giga-ethernet using VEPro5.  Its remarkable the way the machine is working... thanks to 16GB ram in each machine and SSD's, I'm able to run sessions that previously ran on my 2008 mac pro +2009 mac pro slave.  All under 20kg! 

About 3 months ago I bit the bullet, and transferred back to RME audio interface.  I had been using Metric Halo (for portability mainly) but the low latency operation under osx was quite poor, and a number of conversations with other composers led me to grab an AIO as my primary interface, and use one of my metric halo's  (thru adat) for extra channels when I needed it.  Needless to say the change to performance was remarkable - one of the best "upgrades" I've ever had for my system. 

However, now with the portable rig, I'm again facing the challenge of low latency performance in a non-pcie world. 
I am VERY much hoping that the Sonnet or Magma thunderbolt expansion chassis work with the RME PCIe cards. 

I have had a quick conversation to the Magma folk, and they do NOT know if the RME card will run under OSX without some (albeit small) changes to the driver.  They are confident it will run under windows fine with no driver changes necessary.  Given this info, I wonder if it will be the same situation with Sonnet?

Magma are working with a number of audio companies quite closely (UAD, Apogee etc) to qualify their cards... is RME interested in getting on board?  This would provide an AMAZING solution for those of us who require low latency VSTi operation in a portable format.

Thanks all.  Look forward to seeing how this all pans out.

Cheers, Brendan Woithe.