If you notice some slowness, I am playing around with the fantastic New Relic performance and code introspection tool.
I have found that at about 15 - 20 requests per minute per app server, Mephisto starts to bog down a little. We went from a wall clock of 1 second per page view to about 3. Not to bad.
The largest factor was Database hits. Every request touches the database, so putting a simple memcache server into the mix could be a huge win.