After hours with a profiler, a bunch of performance tests and several doppio espressos, the JBossCache optimimistic locking implementation slated for release in the upcoming JBossCache 1.3.0 "Wasabi" is finally looking performant enough for widespread use. While I expect to optimise things even further in this regard, the 3 largest bottlenecks have been taken care of and the performance gains are significant. This is a huge leap ahead, particularly when compared to the performance of the optimistic locking implementation released in JBossCache 1.2.4.
For more information on what's been improved, see this wiki page.
I expect to release a beta for public trial in the near future, but for the impatient, I'd encourage getting the latest from CVS and giving it a whirl.