In what sort of workload was a custom allocator the biggest performance win?

Some more hints: http://events.ccc.de/2008/12/30/the-cat-is-out-of-the-bag/

Here you go: http://www.acmqueue.com/modules.php?name=Content&pa=show...

The issue is that STMs have a lot of book-keeping overhead in addition to the locking.

Wasn't this a targeted attack against a security blogger?

Yes. Under normal circumstances, the attackers would have silently harvested all the victim's accounts and sold them in Estonia.