bush flyer wrote:there is no point in having stepper motors that have 439oz-in of torque
That would be absurd. When I was comparing 0.9° and 1.8° motors, I was thinking about 48 mm long NEMA 17, not anything bigger than that. Typical torques are around 60 oz in for 0.9°, and 70 oz in for 1.8°.
Think of it this way. Acceleration is force divided by mass. You know the mass of the gantry and that of the X carriage. Force is torque divided by arm; in this case, the arm is the pitch radius of the pulley. Take the 60 oz in motor: that's about 0.42 N m. However, that's the holding torque; at higher speed, torque decreases substantially. Take the top speed you're trying to achieve, see what that means in terms of rpm for the motor, and look up the torque in the torque by speed graph from the datasheet of your motor. Let's say it's 0.1 N m. A 20-tooth GT2 pulley has a pitch circumference of 20 teeth x 2 mm/tooth = 40 mm, so a pitch radius of 40 mm / 2 pi = 6.37 mm approx, or 0.00637 m. The force is then 0.1 N m / 0.00637 m = 15.7 N. Assuming 1.2 kg for the gantry, that gives a maximum acceleration of 15.7 N / 1.2 kg = 13 m/s^2, or about 1.3 g. For the X axis, the carriage is much lighter (but don't forget to include the belt and the motor's own rotor inertia); say 400 g to be on the safe side. That gives 39 m/s^2, which is almost 4 g. Of course, there's friction, so the actual acceleration won't be as high; but the nice thing about a laser cutter is that there's no variation in load -- so it will work reliably even if tuned fairly close to the maximum capability of the motors and drivers.
Pro tip: if you want to redo the calculations, Google is very handy. For instance, if you type "15.7 N / 1.2 kg", units and all (but without the quotes), it says "13.0833333 m / s^2". Want that in gees? "13.083 m / s^2 in g", and Google responds "1.33409472 g". The only thing that doesn't work is converting oz in to N m, because oz in is technically wrong -- it should really be ozf in, but Google doesn't know ozf. You need to do it in two steps: "60 oz in lb" gets you "3.75 lb", and "3.75 lbf in in N m" results in "0.423693109 N m". Enjoy!