503

OK. Fixed... many thanks to this post for clearing up a few details.

To eliminate the 503 errors required 3 different config changes:

machine.config:

<configuration>
    ...
    <system.web>
        ...
        <processModel enable="true" requestQueueLimit="100000"/>

IIS Manager > ApplicationPools > Advanced Settings

Queue Length : 65535

and finally (the missing piece of the puzzle), the command line:

appcmd.exe set config /section:serverRuntime /appConcurrentRequestLimit:100000

The web.config setting mentioned in the main post was irrelevant.

10000 concurrent connections, no problems. Thanks for help!