I heard early on that 50 databases was a limit, but I've not seen anyone running that far. I did see this blog (http://www.naterice.com/articles/70) where they mention a 64 bit OS flaking around 100 databases due to thread issues.

5 threads per mirror seems like a lot. You can alter the worker threads, so that might help.

Even with mirroring, I think you want to have log shipping for the delay. That can come in handy.