valid use for gen_server?

[Pablo Platt]

Hi,

I'm building a simple IM server and to make it modular I want to let plugins 
subscribe handlers to messages (ejabberd hooks, django signals, js listeners).
If the handler is sync it can modify the message and return it or return false 
to prevent further processing of the message.
If the handler is async it receives the message but doesn't effect it or other 
handlers.

I see two options to implement the above, gen_event or custom event module.
What bothers me in gen_event is that it executes handlers in its own context.
If events are rare that shouldn't be a problem but if each IM message will fire 
several events wouldn't this flood the gen_event server?

I can use a custom event module that saves events and handlers in ETS table and 
call functions dynamically with Module:Function(Args).
This also doesn't feel right because each IM message will require several ETS 
lookups and dynamic functions calls which according to the efficiency guide are 
are 6 times slower than normal calls.

Can you suggest which one is better?

Thanks