IIRC all typical filesystems for Linux (ext3, xfs, jfs, reiserfs) use directory indexing, usually by means of a b-tree.
It's important to note that these filesystems each have their own strengths, and performance will depend on many factors such as the size and number of files, parallellism, number and type of disks, fragmentation, i/o load, possibly even cpu load. Are we talking about a relaying mailserver or end-user storage? Do the users move or delete a lot of files? Do they rather use imap, or pop3? What other activities run on the machine? How do you see the reliability/performance trade-off?
In real life, things aren't as clean-cut as in most of those generic benchmarks, and people tend to attach too much importance to them and then usually get into silly flamewars. :)
For the sake of this discussion I think it would be most relevant to assume a machine dedicated to Dovecot IMAP Maildir storage without any other services running on it. In terms of load type does it matter whether there are a lot of light usage users or just a few massively heavy usage users? Maximal mailbox reads and writes is probably the best abstract way to avoid the 'how many users / how many mail files?' problem.
Now I believe that people should just pick what they feel comfortable with, because the differences are often not that large and it's rarely worth their time and money.
(Though, that's often not what people want to hear. :) )
Actually that is my gut feeling too. However some people dont' really have a preference and so might as well spend a bit of time choosing the best system in terms of (theoretical) performance for their particular application.