I think you need to consider rpm as well. Without the gear box you have 100Nm, (about 80lb-ft?) but at what rpm and how does the torque decrease as you go to a lower rpm? As an example when you use a face mill often your rpm is low because the diameter is larger; you want a large diameter to remove material as quickly as possible. The problem is you have two things that conflict very badly: The peak torque is at a speed of around 1100rpm I think and the torque becomes less at lower rpms, you may need the lower rpm when you are face milling steel with a large diameter cutter to keep the peripheral speed correct for the material, but the large diameter cutter needs a lot of torque to drive it because the cutting edges are a long way from the center of rotation. Just at the time you need a lot of torque your machine is giving you only a little bit.
With the gearbox I think you will find the maximum torque is at a lower rpm so already it is more suitable for using large diameter cutters, and the torque does not decrease as much as the rpm decreases.
Maybe do some experimenting with small face mills on your MiniMill. I have 1-1/2" (4cm) dia. and 2" (5cm) diameter face mills and can only use the small one with steel on the Super MiniMill. With the larger one I have to use a lower rpm and the machine does not have enough torque to take a good cut.
An open mind is a virtue...so long as all the common sense has not leaked out.