I have to agree with your assumptions...however in this case there are already bottlenecks in the computer.
If you are trying to use all the USB devices at once, then according to the math you should notice a slowdown.
I am just saying that, I have not seen it yet.
The only item I would be worried about is my USB cd burner. But I was also careful with my IDE cd burner.
I have a little network to share the printer, and internet connection. If I am moving a large number of files (or large files) over the network (10M) I get overloads on the Ethernet hub. If I was also capturing video at the same time, I am sure I would see some slowdown...but I get slowdown anyway when I move a lot of data.
I am not saying don't add the other USB port, I think it would be handy, but I am saying that the powered USB hub is just as good for most people's purposes.
On a different note, I have the USB framegrabber that comes with the X10 Wireless Camera.
The camera is low res, but it is light and small, and wireless. The Frame grabber does way better than I would expect. My system again is the bottleneck (400 K6-2 64M ram). But I got a real neat video while the camera was mounted to a remote controlled toy...when I converted it to Video CD and played it on my DVD player, it was better than playback on the computer.