More voltage, not power.
Based on my research, the relationship between voltage and torque is linear up to the max of the stepper. Double the voltage, double the torque at any given RPM. Stepper motors drop off in torque as their RPM increases. That’s why there is always a max RPM with useful torque of a stepper that can be calculated based on the stepper parameters. Variations between spec sheet and real world can also be expected as well.
I did quite a bit of testing with feed rates and acceleration values:
Again, not saying that the 269 oz-in won’t work as it does work (that’s what Phil uses). It’s just not an ideal match (but an ideal match doesn’t really exist either) and for not much more per motor, a better match can be found.
Are you ever going to notice the difference on the Xcarve? Most likely not. You’ll run into other mechanical limits before the steppers become the problem.