My company's site doesn't quite make your definition of "busy" (we peak around 25k hits per day, and 10-15k is average), but here is my experiance anyway: mod_cgi can keep up. We're running several flat-file databases, some very old code written by people who were inspired by Matt Wright, plus new stuff using HTML::Template, MySQL, PostgreSQL, and CGI::Application. So we've got a little of everything. I haven't noticed the production server hitting significant load averages under mod_cgi.

What a lot of people miss about mod_perl is that it isn't just about speed. That's an important point over the inefficient mod_cgi model, but there's something much cooler in mod_perl: being able to dig in Apache's internals. You've no idea how much flexibility this can provide until you understand it.