The stepper motors slip at around 18 lbf, so that’s what limits the maximum cutting force the machine can counteract. You might want to check those limits on your machine. When cutting properly, virtually all of the force on the machine is produced by the cutter’s interaction with the workpiece, so those forces are dependent on the cutting parameters. This calculator will estimate cutting forces for you, or you can use the formulas provided there to do it yourself. You can also estimate force by measuring router/spindle power input while cutting as described here.
Most of stock Shapeoko deflection is likely caused by the Delrin V-Wheels and how they’re adjusted. Good linear rails should have much less deflection. 