**We're using opendjk, not the Sun JDK.** We've been satisfied with the performance and stability of OpenDJ 2.6.0 running on OpenJDK 1.7, which is on version 80, and there have been no announcements about discontinuing security updates. For more information on the OpenJDK project, see their website.
Normally the LDAP server is so much faster than the web services, that it hasn't been a big issue for us. There are proprietary options for higher performance LDAP servers, like ForgeRock's OpenDJ branch, and the UnboundID LDAP Server (which has the same schema and configuration as OpenDJ). Also, UnboundID in particular has a stellar LDAP proxy that can help you distribute the load (and break up large dataests) over many LDAP servers.
Our goal with the LDAP infrastructure was to provide a stable platform that scales to up to six replicas for large datasets (up to 192G of memory per server). Once you get beyond this requirement, you have a serious persistence challenge, and you should get specialized support.