We have always had a great asst waiter/busboy.......However, we dont tip him more than the waiter. I may be wrong but here is my reason...The waiter was probably once an asst and worked his way up to waiter. So if the asst is good, he will become a waiter and command a bigger tip. Would you want someone working at a level lower than yours to make more than you do? I would'nt. Called paying your dues.
You are right about tipping. BUT! Take an extra mintue or two and see the maitre D' and TELL him about the bus boy. These guys are waiting to hear good stuff and taht is how the bus boy gets promoted!