I have built a couple of Ohio's for myself and used the rather severe crescent butt plates.
And you are correct. They are not "shouldered" like most rifles the crescent actually kinda locks right in to the extreme upper bicep area of the arm where it meets the shoulder (the lower "point" of the crescent sits in the armpit. (if you were wearing a shirt with a pocket, the butt plate would be beside the extreme edge of the arm side of the pocket)).
I didn't adjust my typical cast-off on the stocks, but instead of having my cast off line run to the nose of the comb to intersect the center line, I continued it to the rear of the tang (so my wrists are not parallel to the center line of the rifle which I guess does, in essence, increase the cast off. This may cause an issue with the wrist in the future (weakened?), but only time will tell - working fine so far).
I find when you (shoulder) the rifle the upper part of your arm naturally moves more in line with the protrusion of your shoulder where you normally would seat the butt (try shouldering a rifle out where you would a crescent plated one and it will illustrate what I'm trying to say (poorly) above).
It's like your arm compensates for the length of pull which is slightly increased (on your body), by "shouldering" further out from the normal point.
As to recoil, I can't comment at all on that. My Ohio's are smaller caliber (40 is the big one), so recoil is not an issue - don't know if I would build a 50 or 54 shooting alot of powder in that style of rifle - at least not for personal use.