my guess would be that in any server-side application you have to trade performance and bandwidth for "enterprise"-capabilities like fail-over, data consistency over the cluster, message transaction management, scalability and modularity.
so you would either have the first and your system would have reduced or no "enterprise" features.
or you have the latter, reduce performance and bandwidth, and your game would no longer work.
having read most of the dozen-or-so specs constituting J2EE I never read a phrase like "This feature should be implemented to run slow".
if someone would like to make his very own, balanced trade, the choice is given to him by modern J2EE servers. they offer all kinds of optimisations, caches, tweaks and different transaction strategies.
i don't believe anyone stating he can give you unlimited speed and unlimited scalability and unlimited data replication at the same time before he has not put the system under production load (be it J2EE or proprietary). but you don't need /unlimited/ speed and scalability in a production environment, just as much as is needed ;-). and that's what where the deal starts.
"proprietary" engines may be optimized for more-or-less-general gaming needs. but i doubt they can right now be as good as established, well-known J2EE engines when it comes to "enterprise" features, can they?