And you think that high speed communications use the old style asynchronous start bit, stop bit, data link layer communications protocols? Without knowing what sort of high speed protocols they're using at the physical and data link layers, we can only guess (probably based on ATM style cells), but for the link directly to the computer, you've got IEEE 802.3, which gives you between 61 and 1497 bytes of data (don't forget the 802.2 LLC header), and the overhead is 8 bytes for the SOF delimiter, another 4 for the CRC, and another 12 for the MAC address.
Mark
And you think that high speed communications use the old style asynchronous start bit, stop bit, data link layer communications protocols?
Probably not but, as you say, there's transmission overhead involved in any protocol which makes the divide-by-8 for converting bits-per-second to bytes-per-second incorrect in any case.