You can set the MALLOCTYPE=watson environment variable, available in AIX® 5.3, for use with the IBM® 5.0 JVM, but for most applications the performance gains are likely to be small. It particularly benefits any application that makes heavy use of malloc calls in native code.
For more information, see this article: http://www.ibm.com/developerworks/eserver/library/es-appdev-aix5l.html.