Smoke,
I'd be willing to bet that the top pikers at IB do as well, if not better, than the top performers at many of the so called professional shops. They may even do more volume.
But you'd be comparing apples and oranges as many of IB's clients are a who's who's list of professional fund managers, FA's, CTA's, investment banks, hedge fund managers, ex-floor traders and some fantastic traders sitting at home and trading on their own. IMO, The larger the account, the less likely one would want to go pro. IB's speed of routing, if not the best, is amongst the best industry. Basket utilities, API"s and guranteed VWAP's are not the tools of scrubs. Even if these higher net worth accounts don't choose IB, they would seek other alternatives besides a prop house.
Anyway, no offense taken by me. To get back to your question. IB self clears, has a prop side that is consistantly amongst the most active traders world wide, is almost completely automated and even provides access to some other brokers/houses on a give-up basis. Most prop/prof firms and other brokers are not self-clearing, don't own the exchange seats, and do not have the economies or efficiencies of scale to offer the same rates as IB. Simply put, IB can do it at a lower cost.