The top first configuration is not uncommon. The widely held belief that the top barrel recoils more led to old days pigeon shooters using 1¼x3¼ loads in the 1st (top) barrel and something stiffer in the 2nd (bottom) supposedly lighter recoiling barrel.
I've never heard anyone say that the top barrel
recoiled more than the bottom. Not saying at all that some may have claimed that, just that I've never heard it. There is, however, no question that the top barrel causes more muzzle rise than the bottom. Maybe that's what they meant by recoiling, but I just understood the term differently. The physics is exactly the same as a S x S recoiling right when the right barrel is fired, and the left recoiling left when it is fired. As has been discussed recently, this is the reason for barrel convergence with S x Ss. The top barrel is farther off centerline axis than the bottom, causing the extra muzzle flip.
I always set up my O/U guns to fire the bottom first for pigeons, game and sporting clays. It may not make but a fraction of a second difference in how quickly you can get off the second well aimed shot, but why not take advantage of that fraction of a second? I'll admit it's not much, but it is real, not imagined.
SRH