Yeah. Modern (but not bleeding edge) optocouplers can do 100ns pulses which would give you 10mbps - but I’m not sure how that plays with the failure mode for the transmissions. Ethernet doesn’t care how many packets your sending per second, it’ll try to do the full speed from the get-go, so the question is whether the errors would be exacerbated or alleviated by retransmission from higher up in the stack.