Although I don't have any personal experience with the Combat motor, from what I have heard they are a little more peaky in the power delivery and much less reliable than the 850. You can punch up the 850 with a Megacycle or similar cam, and clip the head a bit to bump the compression and get performance close to or equlivalent to the Combat, and be more reliable at the same time.
That is what I did with my 850, just be careful that you check the piston to valve clearance if you go to higher lift cam. My pistons needed to be pocketed to make sure that the clearance was right.
My motor will spin to 7000, pulling strong all the way and have plenty of torque above around 3K. I haven't ridden any other Commandos, so I don't have a Norton comparison. However, I can compare it to the 94 900SP Ducati I used to own and the Norton's acceleration, while clearly slower, is not that far off.
Bottom line is that I would stick with the 850 and just hot it up a bit if you want some more performance. You probably would not be happy with the Combat reliability from what I have heard.