Question: why are you going with FastCGI over mod_perl? Issues with mod_perl debugging/memory leaks? Desire to use nginx over Apache for higher throughput? The big advantage -- to me -- of mod_perl is the power of having your code live within Apache's process space.
Initially I didn't think anything could beat mod_perl speed, but after benchmarking, this setup is just as fast. It's also more stable, and uses less memory. And yes, also it is easier to debug and I was getting memory leaks on mod_perl.
Don't get me wrong, I love mod_perl. I used it at my last company and on many other projects before this one. And I still use it for other stuff.
Last I looked I think you can run mod_perl with event_mpm but I didn't like this: "The current Apache::DBI should be usable under threaded mpm, though it doesn't share connections across threads. Each Perl interpreter has its own cache, just like in the prefork mpm." (http://perl.apache.org/docs/2.0/user/performance/mpm.html)
I wonder if mod_perl works with event_mpm, yet.