I have them on two rifles currently but have used them for years. You can actually get 40MOA by using the inserts as described in the installation manual. As stated, they leave no marks at all.
They do however, take some time and patience to set up properly if accuracy is the goal. And yes, if windage is off some, they can be twisted from horizontal to gain that center as needed (also explained in the installation manual).
The Signature XTR is the best option in my opinion.
I normally shoot a large cardboard at about 25 yards while it's in a vise, then adjust the scope to that point of impact to get very close, lock it down and make fine tune adjustments from there.
Mike