> the standard minimums are 40ms tone + 40ms interdigit silence,
> or 160ms/byte - at which rate it could be a while before I see
> the end of that 500 byte transmission.

You're skating on the hairy edge there.. 50mS on, 50 off is standard.
Pushing it any faster than that calls for some FEC to make sure you don't
have to rerun the whole packet.