In my own experience a pistol grip will do more to determine hand position and influence the feel of stock length.
On a straight grip gun things are a little murkier. Are we assuming that length of pull is still being measured from the butt to what, the front trigger? If that is the case, the hand will be set quite a way back from the front trigger, having to clear the rear trigger and the back end of the trigger guard. And with a straight grip, there's nothing to prevent the hand from being placed so far back. No small co-incidence that so many of the British and Continental straight grip guns have pulls on the order of 15". I think that is why CSM wisely offered the option of a longer stock.
