Most of the answers provided here address the number of incoming requests to your backend webservice, not the number of outgoing requests you can make from your ASP.net application to your backend service.
It's not your backend webservice that is throttling your request rate here, it is the number of open connections your calling application is willing to establish to the same endpoint (same URL).
You can remove this limitation by adding the following configuration section to your machine.config file:
<configuration>
<system.net>
<connectionManagement>
<add address="*" maxconnection="65535"/>
</connectionManagement>
</system.net>
</configuration>
You could of course pick a more reasonable number if you'd like such as 50 or 100 concurrent connections. But the above will open it right up to max. You can also specify a specific address for the open limit rule above rather than the '*' which indicates all addresses.
MSDN Documentation for System.Net.connectionManagement
Another Great Resource for understanding ConnectManagement in .NET
Hope this solves your problem!
EDIT: Oops, I do see you have the connection management mentioned in your code above. I will leave my above info as it is relevant for future enquirers with the same problem. However, please note there are currently 4 different machine.config files on most up to date servers!
There is .NET Framework v2 running under both 32-bit and 64-bit as well as .NET Framework v4 also running under both 32-bit and 64-bit. Depending on your chosen settings for your application pool you could be using any one of these 4 different machine.config files! Please check all 4 machine.config files typically located here: