Hi James.
I've not built any linisteppers, but It's a very clever design. It's on my list of things to play with when I get some time, but I keep on getting other projects leaping into the 'queue' as it were.
re: speed. I think the real question is how fast a stepper has to go for the application, with that figure in hand you could make all sorts of rash suggestions!
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Cheers, Chris H.