Firstly, when Matrix returns binary files to the browser, is this being piped through or return output from matrix's PHP scripts?
If so, does the implemtnation of the System Static URL mean that the file requests are coming directly from the webserver? which in turn would allow byte-serving of PDF files if optimised to do so?
- No, its not. By default, Matrix rewrites the URLs for binary files that are Live with Public Read permission to use /__data location. This in turn is aliased in Apache directly to the mysource_root/data/public file location. So, Apache will natively serve those files. If you uncheck the "Allow Unrestricted" option for a file asset, Matrix does not change its URL and it is piped through PHP. However, it retains its "friendly" URL.
- The System Stactic URL allows you to offload the serving of pure binary files to a different webserver entirely. All you need to do is make sure the Static URL you provide is serving files from mysource_root/data/public. We use lighttpd in some high-load situations to offload processing from Apache.
Excellent my main concern is that the binary files, especially PDF's were not being served natively by the webserver… which would cause the Page at a time or byte serving to stop working for some of our somewhat larger PDF files.
But all good. 
Which I had no doubts on