Do some searching on this forum to find out what USB version your unit has. If it is a newer unit, it may use USB 2.0
For USB 1.1, the throughput is 12Mbps
For USB 2.0, the throughput is 480Mbps
For a wireless B connection, the transfer rate is 11Mbps
For a wireless G connection, the transfer rate is 54Mbps
For a wired 10/100 connection, the transfer rate is 100Mbps
Now, looking at those numbers, your transfer speed will be equal to the lowest number in your chain. You can't push more water through a pipe just cause the next pipe in line is bigger.
Also, even with USB2.0 and a wired 10/100 connection, your data transfer rate will likely still be below the max of 100Mbps. The TiVo has to process the data it sends and processing slows down the transfer rate, however, a wired 10/100 connection on a USB 2.0 TiVo will still be the fastest.
Jeff