It depends on the JVM release. On my Windows 8.1 laptop running 1.8.0_31,this flag increases AutoBoxCacheMax from 128 to 20000 (which affects autoboxing and collections performance), and it lowers BiasedLockingStartupDelay from 20000 to 500 (which can effect multithreaded access to shared data - see http://www.oracle.com/technetwork/java/tuning-139912.html#se... for more details).

It isn't strictly necessary, but if you're trying to squeeze every possible drop of performance out of your Minecraft server, the experimental optimizations aren't a bad thing to enable. I will admit to doing only informal performance testing when it comes to the AggressiveOpts flag.