4 September 2013 at 11:44am
(Last edited: 4 September 2013 11:51am),
The problem you'll face is the URLs for uploaded files/assets in SilverStripe don't support a subdomain the way you're wanting. You would have to modify the way the assets path is set in the framework.
If you're wanting to increase performance, specifically for static files in assets, I would look into using nginx for serving static files and proxy pass PHP requests to Apache.