There isn't a simple answer, as there are many ways to send messages. For example, you can send messages synchronously or asynchronously. As a rough benchmark, on the dual 66MHz BeBox you can send about 2,000 messages/replies per second. Of course, this will vary with the time you need to spend actually processing the message(s), but raw sends/receives are fast.
You can send about 10,000 32-byte messages/sec on a dual 66Mhz Bebox using the underlying port mechanism.