I see we never published benchmarks on that. I'll ask Vadim if he still has the numbers and can blog about it. I don't want to quote something from memory and be wrong, but I'm pretty sure the performance hit was into the double digit percents, which is a non-negligible amount of money on large applications. Although it's possible that newer distros are improved in this regard.
CentOS 5 and Percona Startup (and my.cnf feedback request)