8 wire steppers, parallel. series or single winding?

I know this has been addressed before but here goes any way. I have some 8
wire stepper motors, Pacific Scientific NEMA 23's, and I would like to know
what configuration will give the highest speed. I am currently running them
in a parallel configuration which gives plenty of torque but seems to limit
top speed. Would I get higher speed by running single winding per phase at
the cost of torque?

Thanks

John Guenther
Sterling, Virginia