What is optimal IIS threading settings for a Win2012 server that does lots of slower web service calls?

Question

We have a Windows 2012 Server that communicates to lots of other services via SOAP based web services using ASP.Net and .NET 4.5. These services tend to have higher latency (i.e. 2-6 seconds to return). We believe we have threading issues since load on the server will start generating 504 Gateway Timeouts. What is the optimal threading configuration for a server that communicates heavily with high latency SOAP based services?

Note: All the SOAP calls are synchronous and we can't change that unfortunately. No Async

Here is the default in IIS8/Win2012

<system.web>
        <applicationPool maxConcurrentRequestsPerCPU="12" maxConcurrentThreadsPerCPU="0" requestQueueLimit="5000"/>
    </system.web>

Two reference articles that i have read through.

http://blogs.msdn.com/b/tmarq/archive/2007/07/21/asp-net-thread-usage-on-iis-7-0-and-6-0.aspx http://fullsocrates.wordpress.com/2013/02/28/asp-net-threadstuning-thread-parameters-22/