tweaking under repeatable load

I'd like to optimize my postgres configuration for optimal
performance under typical load. Unfortunately, as I understand
things, that implies that I have to have a way to repeat the same
load each time I try out new settings, so that I can fairly compare.
It's difficult for me to drive the load all the way through my
application, so I'm envisioning some script like pqa that analyzes
the logged queries against the database during a typical load period,
and then lets me replay them directly against the database with the
same original frequency.

Has anybody done anything similar to this? Anything better? Did it
end up being a waste of time, or was it helpful?