A little over a year I ago, I made a post to the Asterisk-dev mailing list looking for someone to do some load testing with the Hoard Memory Allocator.
Chris Tooley just recently gave it a test run and saw a boost of about 10% in the number of call setups per second that his machine could handle.
I would encourage others to give this a try when load testing your system. There is virtually no setup other than installing hoard. There are some Asterisk functionality that I feel would see much greater performance improvements than have been demonstrated so far.