Hi,
all steppers lose torque the faster they go, thats plain physics and you can't get around it.

The inductance of a stepper is a good measure of how bad that torque degradation will be, the lower the better. For example a 269oz.in stepper of 5mH
inductance probably will have 5% or less, ie 13 oz.in at 1000 rpm, whereas a 260oz.in stepper but only 1mH may have 100oz.in at 1000rpm.

I think your 23 size 269oz.in steppers are about right but they MUST be low inductance or you will be disappointed.

The second way to defeat the degrading torque is to use the highest voltage drivers and power supply you can. Don't waste your money on a 24V supply,
that wouldn't pull the skin off a rice pudding, and those TB6560 drivers have a poor reputation for lasting. What you want is 80VDC capable drivers,
about the highest voltage of regularly available drivers with an 80VDC linear (transformer/rectifier/capacitor) power supply.

Craig