Using different http connectors - not the http connector for the BPM app - for your applications is HIGHLY recommended. That way, when http traffic for your one application goes over the top - you do not bring down your entire BPM system.
Using different thread pools - per http connector may be useful if you have several request-response type invocations that will result in threads being consumed for longer periods. By design, this is not a good idea, anyway. You should strive for async calls in such scenarios.
Apart from tuning of the database connections, PE threads, LDAP connections and http timeouts - you have to run performance tests to see what will make a difference for your environment. That's the best way to determine if making tuning or changing specific application level parameters will work or not. Every application behaves differently and needs to be tuned accordingly.