I have done mostly aluminum on my x3 so far. But the couple of pieces that I did cut mild steel was about .040 and .050 doc a pass. This was full width of cut on a 3/4" 4 flute endmill. Tin coated. I think the rpm was about 450-500
I didn't have any problems with stress and I only needed .060 off total so I didn't get to crank it up. I need to make some new brackets for my tractors front end loader soon, so I can report back when I start them. I would guess that about .100 would be max and that I could get 10-15 ipm out of that size cut.
I have done .050 doc with the x2 mill and a 1/2" 4 flute endmill without any problems. Obviously that was a rough cut. I would rather compromise between really fast small cuts vs deep slow cuts. I just take the middle ground.
Don't worry about asking questions. It is good that you have done your homework. We can tell by the questions you have asked. It is easy to see that you are going to enjoy this machine and put it to work.