Morm,
Have you first extended the number of maximum simultaneous requests Abyss Web Server could handle? This parameter is documented in
http://aprelium.com/data/doc/2/abyssws-win-doc-html/servergeneral.html#SERVER-PARAMETERS-MAXCONN .
How are the tests done? Are you downloading a single large file or are you using a tool that will make several requests to your files? How large are these files?
By the way, do you have an antivirus or any router (with content inspection) or a firewall? These are usually a large bandwidth limiter when you host a server.
Here is an old document about tuning Windows TCP/IP implementation:
http://smallvoid.com/article/winnt-tcpip-max-limit.html . But keep in mind that we don't recommend fiddling with them unless you know what to do.
Also knowing how you're trying to saturate your connection is usually a key to understand why you're experiencing this limit.
Also, keep in mind that most routers "favor" download and are not well tuned for uploading data. They are favoring most consumers behavior who download more than they upload.
Oh, by the way, your ISP is probably also capping your upload speed. A saturation test has to be done locally first to test if your network card does really handle on its own the speed before moving further.