The biggest effect on the center rigidity is distance of the tool tip from the bottom of the gantry. Half the distance, twice the rigidity.
When you say you are having a speed limitation how fast are you going now, through what material, how far from the gantry is your tool tip, and how fast would expect a machine under $10k to be able to go?
I mill aluminum with the tool as high as possible, I actually have a new mount for the dw660 that should make things even more rigid but At the moment requires modifications to the z rails.
No comment on the rail, at that point you are making a different machine.
Why switch to a lead screw? Plunge rate through most materials is really slow, seems like a waste to change all that just to speed up your z rapids.