llandy - Sorry, I was trying to be as accurate as I know how (apologies for my 2.5 vs 2.25 gaff)
I always learn something from these discussions, partly because I have to reexamine my old assumptions, so I am grateful for your forbearance, putting up with me being pernickety.
I think we nailed this one :-)
Aside - I don't think most OS's could cope with 2.25mbps, with a traditional serial interface. I hadn't thought about the implications until now, but I wonder if maybe that's why USB was designed to pole the devices; to make the interface electronics cheap on the host side?