Probably a dumb question, but how is it that a RapidRoad connected to an X-Surf 100 in ZIII can get up to 6-7MB/sec (source: http://www.fitzstevesamigaworld.co.uk/?p=253 ), but Ethernet only 2.2MB? Is it all just protocol overhead? Seems like 2.2MB/sec isn't a bus limitation if RapidRoad can do so much faster through basically the same pipeline. 
I know very little about how USB actually works, so this is a bit of speculation.
I reckon that the differences come about due to how much work the USB hardware does (as opposed to how much work the CPU has to do in addition to processing the incoming/outgoing data on Ethernet), how much freedom the implementors of the USB stack have (as opposed to the abstract "one size fits all" approach which the Amiga SANA-II network driver standard imposes) and how complex the underlying data transmission protocols are.
The type of Ethernet hardware we have to use for the Amiga is basically a data transport device, which sends or receives data which then has to be processed by software that, for example, has to verify or recalculate checksums, pick the structure of the data apart and make sense of it. This is by design, and not a side-effect. The CPU not only spends time on looking at the data, it also has to copy that data around 2-3 times between the network device and the application software using it. This adds up quickly.
In addition to shuffling the data around that comes and goes through the Ethernet device, the set of protocols which govern how the data flow is adjusted to the available bandwidth, and which detects and handles retransmission of missing data, is performed by the CPU as well. These protocols add a substantial layer of complexity, which is required by the fact that the data has to be able to travel a long way through several intermediate systems if necessary. So there is a lot of slack and elastiticy in these protocols to provide for this robustness, and that is just the opposite of what you would want in a fast data transmission system.
USB only has to cater for a local data bus which attaches peripheral devices such as hard disks or mice to your computer. There is no need to support mice connected to your computer by 10 km cables
Getting the data where it needs to be is also a lot simpler: the source/targets are all located on the same bus, whereas for TCP/IP the source and target could be anywhere on the local network or even the Internet.
Also, USB is likely designed so that the heavy lifting (more than just the basic work of doing error detection, retransmission, etc.) can be handled completely in hardware, with the computer tending only to the mandatory operations, such as handling device detection/removal, changes in the topology of the setup, and of course exchanging data between the computer and whatever is dangling out there.
Because there is nothing like the SANA-II networking standard for the USB stacks, and which today forces Ethernet hardware drivers to ignore all technical advances of the past 25 years, the USB stacks available for the Amiga can talk to the underlying hardware in any manner which they see fit, and which is likely best suited to the task at hand.
Now if you hooked up an Ethernet device to your USB setup, you might see that the overall throughput won't be in the same league as the basic USB data throughput. It would probably be a lot worse than what the X-Surf 100 can do all by itself.
