P [W]
Stands for maximum (instantaneous) mechanical power of a motor when going at full speed and utilizing full torque.
This wont happen in real life as with stepper motors you'll have a safety margin of 1/2 so you will get only half of that power from a stepper motor. It's also important to bear in mind that you will utilize half of this power only in two instances. First instance, when you're accelerating to full speed at full allowable acceleration during fraction of a sec just before reaching max speed. Second, when cutting at full speed with full allowable force. Otherwise you're using only fractions of that power.

Cutting a steel is done between 1 and 2 m/min so if your Vmax is 10 m/min than you will use only 10-20% of Pmax and only if the gantry had to be pushed with full allowable force which is unlikely to happen.

In a cell where you input a torque of motor it was required to input a combined torque of motors for given axis. Now you just input a torque of motor and number of motors.

I also see that you are interested in power of motors, so I've attached one more spreadsheet inside to calculate various mechanical powers of motor, as well as some information on power and current drawn from power supply - to help choose PS.

Attachment 11075

I don't know much about application of servo motors but from what i briefly read, the most important feature of a Servo Motors is a possibility to overload them up to 10 times which is not possible with stepper motors due to the nature of a driver.

If this overloading feature is possible than consider following.

For a 400 W servo motor that you've selected (assuming an overload factor of 5) you can get as much as 2000W for a fraction of a sec.
Now, if you open my spreadsheet (the new one) all the data there are for my design.
Equivalent Gantry Weight 290kg
Motors: 2x 1,5Nm -> maximum power of 150W per motor but steppers have to be used with some safety factor so as not to loose steps.

I've taken 1/2 so I'm Using only 75W of each motor. With that torque/power I can accelerate 290kg gantry to 10m/min (393 inches/min) in 55 mili sec. That's quite fast but with your servo and 2000W you will be able to accelerate it to the same speed in as little as 4,5 ms over a distance of 0,38 mm, ceteris paribus.

So what I want to say is: don't over-invest in motors.

Best regards