[PAR_WIG1350]

I'm thinking something like a tapered cam on a stepper attached to a lead screw attached to a second stepper might work for adjusting the stroke in one direction, while allowing for some very high speeds. Varying the stroke in the other direction would require more complex motion and would slow it down a bit, but it would still be fast.

EDIT, It would probably be faster to use a standard cam and drop the lead screw and second stepper. The tapered cam would be better only if you had a set return point and only one end of the stroke varied.