Aha! You've figured it out!
Actually, the "better performance" is better ability to handle temporary connection interruptions, not overall bandwidth as such. Since there is more "elbow room" in the potential 54Mbps max, there is a greater capability to resend fragmented packets, thus improving the overall appearance of the stream.
An ideally configured 802.11b network, one without dropouts, dead spots, and interference, should have no problem dealing with any streaming video that your cable modem can handle. Too bad this ain't a perfect world.