apache 2.2 mod_proxy_balancer limitations?

I'm currently testing a setup on a dedicated server with Apache 2.2
on CentOS 5 with mod_proxy_balancer and mongrel.

Static files are being served at around 1200 req per second (by
apache) or about 600 per second by Mongrel.

The problem is dynamic pages. I can't break 160 req per second
regardless of number of mongrel processes (2-12 all yield about the
same). It's a simple "hello world" controller.

Then I found this at the bottom of th Mongrel docs:

"Jason Hoffman reports:

    Apache's mod_proxy_balancer module is a fully blocking module and
with the default httpd.conf you're going to max out in the 120-160
requests/ second range on a decent box. You can tune up its proxying
to about a 1000 req/sec. So yes the net result is that you can really
only put a couple of mongrels behind apache's proxy engine (about 2
"hello world" rails mongrels)."

Of course it doesn't elaborate any further. I searched around google
and found nothing. Is this really the best performance you can get out
of it, or exactly what should I be tuning in httpd.conf? I find it
hard to believe that the benefits max out at 2 mongrel processes...

Thanks for any help