How do I reduce the threads per process limit for an ISAPI app under IIS 7.5

How do I reduce the threads per process limit for an ISAPI app under IIS 7.5

I have a python WSGI application running under an ISAPI extension in IIS
7.5 on Windows Server 2008. I would like to increase the number of worker
processes (which I know how to do) and reduce the number of threads per
process (which I don't). I'm concerned that I may have too much contention
for the GIL.
I have seen instructions such as these for modifying threads per process,
but that setting doesn't seem to affect my application - my w3wp processes
still show the same number of threads. I assume this is because that
setting only affects ASP.NET applications, which this is not.
I'm aware that I could switch to a different python runtime such as
stackless or twisted. Or a different host such as Apache. Or a different
OS such as Linux. I can't afford those tasks right now, and am looking for
some shorter-term tuning.