This is an educated guess but believe it is a toss up between the leadscrew and the power supply voltage.
A different leadscrew will definately help. However, replacing a leadscrew is a lot of re-engineering and work with the bearings, motor couplings, etc.
The 24 VDC supply is probably the least expensive route to take as it will also give you additional speed in rapids. Also, it is probably the least amount of work to change out.
Steppers with higher inch/oz rating should not make any difference in speed. Just in torque. However, higher voltage should also increase the torque rating at the present speeds.
Hope this helps,