Your servo driven machine with closed loop controller isn't an apples to apples comparison to an open loop no-feedback stepper setup. Also in the same description, you describe your 140 oz-in servo setup and go on to state that the same machine uses 1200 oz-in steppers. This further indicates that the servo comparison isn't valid. While I don't argue that your knee mill table is heavier, moving the table isn't the only consideration. Machining loads and machine vibration are further concerns. A machining load for a given size cutter cutting a certain depth and width of cut at a given feed and speed will be the same regardless of the size of machine that is doing the cutting. My experience also suggests that the smaller less rigid machine can actually create higher loads on the machine due to vibration.
Also remember that I plan to move these steppers over to a bigger machine later. A machine that has a table nearly as big as a knee mill, and that will need to move the entire head for Z-axis operation. I understand what you are saying about the larger steppers and the speed limitations, but moving the table at super high speeds is not a concern of mine for a hobby machine, and the acceleration profile isn't a concern since these size motors obviously work for milling on larger machines. So the acceleration/deceleration ramp is more gradual with the larger motor. I can live with that.
It seems the answer to the question of whether the 880 oz-in will work in direct drive on my current mill is a yes. Also answered is that these motors will work for both the small mill I have now and the larger one I would like to get later. I guess the only question on the larger mill is if the 880 oz-in motor has enough to drive the Z since the deal I have found is for a set of 3 motors, but if I find they don't have enough, I can go to a counterbalance setup or gas strut system to help them out. Additionally, I can go to a reduction belt drive as well.
Thanks again. I welcome more opinions.