Hi Mike,
seems like you've concluded that the mechanical limitations of your machine make it impractical to markedly increase its performance.
Without replacing the lead screws I tend to agree.
At the end of last year I started using an external controller, an ESS. Amongst the things I have noted is that my steppers are not as noisy,
or rather that the noise is very much less harsh. Also its possible to push them somewhat harder to accelerations and speeds which would
have caused a stall with the parallel port. These are consistent with improved pulse stream timing available from a dedicated controller
rather than your PCs CPU which has hundreds of tasks on the go at any one time.
An external controller may be a worthwhile improvement to your machine, in your case it means a controller, a BoB and either a combo
or individual drivers, not an insignificant investment. If they are chosen wisely however you will certainly be able to reuse them in your next
machine, and your post suggests you are thinking about it,... have you told your wife? I suspect with decent high voltage drivers and supply
your existing steppers will surprise you.
Craig