Doing processing in Java will cause a lot of network traffic between the database server and the app servers. If I were to create a similar system I would want all of the logic for accessing data to be in PL/SQL and all of the presentation code to be in Java. Java should make a specific request and PL/SQL should provide the data or do the DML on the database.

IMHO, Web apps must be scalable to work properly. If you wrote stored procedures that are slow, then either they need to change or you need to tweak you schema in your database to get better performance. Small inefficiencies in code in this environment can create big problems with scalability.