mod_perl is typically 20-40x faster than vanilla CGI.
Uhm, that's a bit of useless statement. It may be true in some cases, but it isn't true in other cases. One should consider where the use of mod_perl gets its gains. It gets in gains because it can share some resources, which includes process space and compilation. Another important resource where it can save is sharing of database connections. So, if you have lots of little CGI programs, each doing a quick job, the use of mod_perl can save a lot of resources. If you have CGI programs doing a relatively long jobs (say you have some programs that do custom image manipulation and that process dwarves the time needed for compilation - or you are doing database queries and the time it takes for the queries is far more than starting the Perl program and setting up the database connection), the savings are minimal.
So, I'd say the answer to the original question would be more like "NOT ENOUGH INFORMATION - DOES NOT COMPUTE". And my answer to the PHB's first question would be "You haven't heard the whole story - in some cases it will save big time. But if you give me a project code, we can run some tests to see how much it matters. BTW, extra hardware also means more reliability."