You seem to misunderstand the purpose of the cache... the cache is server side, meaning that the UBB scripts themselves use it, not the clients. The UBB just sends the premade cache page to the user rather than generating a copy from scratch. This will use the same ammount of bandwidth as doing it any other way.
Some hints... turn off the iframe on the reply page, use 15 posts per page for the threads and 25 topics per page in the forums. Try and eliminate any graphical header or footer, if you have any...
