I hope to be able to use lathe as designed, ie, down to 4 tpi, with 1/2 nuts in, doing real work in CS1040, or 40 ton steel. Will drive as U r, thru the gearbox. I think I would smash gbox if I stalled the leadscrew too, (no safety clutch provided)I will try to provide some kind of answer to that question in the next few days. Assuming that you would be driving through your gearbox, you would have some leeway by virtue of the ratio selection available there. Work backward from the maximum pulse rate that the ELS could output and scale things from there. You will always have a practical limit on how fast you can spin the leadscrew. I detect when I can't keep up and show an error, or the driver itself shuts down. I just tested it and with the lathe running at 210rpm (no load) I was able to get to 5 1/2 tpi before the driver itself crapped out. It would be sooner with the half nuts engaged, but it would be trying to move the carriage about 40 inches per minute. When that happens I have to turn it off for a minute to reset. Must be a thermal protection circuit. So far it's always come back. So far.
But as KB said earlier, lighter cuts probably.