I should mention that I've found a couple of patterns useful in tracking down
performance bugs:

darcs obliterate --last 100 -a && darcs pull -a

darcs unrecord --last 1 -a && darcs record -a -m foo

It would be helpful for a benchmarking code to focus on inverse pairs like this,
because they're very easy to work with when tracking down code.

And a good start for a benchmarking framework would simply be something that
sets up a few repos and then runs the above on each of them.

David