Speaking from the perspective of a USB hardware designer, your rationale for plugging your mouse into the MB directly so it has max bandwidth isn't quite correct. USB traffic is separated into 1ms frames using a special SOF (start-of-frame) packet which helps keep all the devices on the bus in sync. ALL USB mice are low speed devices (meaning the data transfers at 12MHz) and by definition the fastest they can be polled is once every 10 frames. Additionally, all low speed USB devices on your system will share 10% of the total theoretical bandwidth of the bus. I think this would include your joystick and radio though I am not positive how those devices are designed. High speed devices (your camera, zip drive, plus scanners and printers) transfer their data at 48MHz and must share the remaining 90% of the bandwidth. Some devices request a specific percent of the bandwidth be assigned to them for streaming data at a constant rate (web cams especially) while others will dynamically use what is left over.
Gee, that was more info on USB than anyone probably wanted to read. What was my point? Oh yeah, since your mouse uses such a small percentage of the bandwidth, the only performance gain you get by not using the hub is whatever the hub's delay time is in propagating the signal. For a mouse, I'm fairly certain that not even the most elite Quake III player could notice the difference.
And to address the original poster's question, there is a theoretical maximum of 12mbps of data transfer available to ALL USB devices plugged into the same root port. As long as the devices are designed well, they will negotiate the best possible way to share that limit but yes at some point if you try to scan, print, run a streaming web camera, etc all at the same time, at least one of the devices will have to run slower than it is capable.
BTW, the two USB ports available on most modern motherboards are actually connected to a hub built into the motherboard. Thus all devices connected to BOTH of those ports must share the 12mbps maximum bandwidth. The only way to have more bandwidth is to buy PCI cards with additional root hub controllers on them.