[Exim] Filter Efficiency

Before I get to my question I have to say that I am very impressed with
Exim. I have a Smail based server here that I've HEAVILY customized
over the years, but configuring a replacement box with Exim that handles
all of our specialty services took only a few hours. Well done gentlemen!

My question is regarding global or central filtering. How efficient is
it? Does anyone have any stats to share?

I'm looking at a box that will shortly handle about 100,000 deliveries/day
and about 3-4Gb/day. I'd like to reject certain obvious spams that we
happen to see a lot of here (eg: certain X-Mailer contents, friend@public
in the to: header, etc). I know it probably depends a lot on the filter
itself, but let's say it just performs a couple of dozen header content
checks and that's all.

Mark