First off, I apologize for not responding to this thread earlier. I did not have an account at the forum and recently created one.
Service Broker can basically be used to achieve asynchronous semantics in the database. In your case, the application tier receives messages from an external source and if you are not using transactional messaging in MSMQ, the performance of SSB and MSMQ is comparable. Also, in your case, if the application tier is load-balanced, you will have multiple Q's doing the job. I would not recommend replacing this solution with SSB because, if you need to do some pre-processing of the Q before executing the SP, then you cannot do it. Also, by using MSMQ in this case, you can have other applications that can also drain the Q.
SSB is largely for writing reliable, asynchronous, message oriented database applications.