CgiApp will have a smaller footprint as it uses less modules and, frankly, does less. That said, the Catalyst devteam is very keen on scaling. Now, a major piece of this is how you deploy. Most CgiApp deployments that I've seen/heard about are done under mod_perl. Catalyst tends to deploy under FCGI. That can make a huge difference in your overall memory footprint (MP is often better) and scalability (FCGI is designed around scalability). As for HUGE number of hits, I'd say they can both handle it.

And, if expressivity wasn't important, you'd be using C and not Perl. Plus, Lisp often beats out C in the Language Shootout. We don't use Lisp because it's not fashionable enough to have a big enough ecology around it. Perl is Lisp minus a few features, but plus CPAN. Perl 6 is meant to be Lisp + CPAN, no minuses.