A couple of points; the std English feild or game load is 1 1/16 oz. 1 1/16oz times 96 eqals 6lbs 6oz. 1 1/8oz times 96 equals 6lbs 12oz. 1 1/4oz times 96 equals 7lbs 8oz. The 2 1/2" shell std loading is 1 1/16oz, the 2 3/4" std loading is 1 1/8oz and the std pidgeon load is 1 1/4oz. This is why English game guns gennerally weigh between 6 1/2lbs an 6 3/4lbs and pidgeon guns 7 1/4lbs to about 7 3/4lbs.
Virtually all American guns were built to the pidgeon weight. Try going into a dove feild and finding the average charge being shot at one of our very smallest game birds, it going to be over 1 1/8oz! The ammo catologs are full of heavy weight loadings, while the best game loads, waterfowl excepted, are found in the target ammo selections.
On hand movement I actually believe you are both right. I think the hand moves some with a straight grip but then a straight grip gun is pointed by the forward hand, or at least should be. The full pistol grip is best suited to pre mount were the mount and point is replaced by merely a swing. I also think the full pistol grip and the way it is more firmly grasped does help with recoil absorbtion.
For game shooting of all types I subscribe to the ideal being that the swing and mount "conclude" when the gun reaches the shoulder and the trigger is slapped and the swing followed through. The straight grip rules here. A pistol grip leads to too much influence from the trigger hand and this is not ideal.(I'm a lefty thus no mention of right hand or left hand only the position of the "trigger hand" or "leading hand"; I get confused when trying to translate for a righty!)
I don't believe that the style of grip leads to higher or lower shooting believing that drop at comb and heel dictate this allong with some influence from pitch.
On recoil, I have shot upwards of a thousand rounds a day for days straight at doves, pidgeons and ducks in Bolivia and Argentina with my 12ga straight and splinter gripped 6lb 12oz Arrietta "travel" gun. I'd have prefered shooting light 1oz or 7/8oz loads but the outfitters never carried them since they try to keep their shell inventory limited to shells which will cycle any auto loader even when that auto loader is getting dirtier and dirtier through the course of a day so the 3dram equivelent 1 1/8oz loads are what I shot. Recoil was never an issue with me because my gun fit. I've seen dozens of guys reduced to black, blue and red mush with their auto loaders that didn't fit well. The first time I went south the gun had a checkered butt and the friction of the butt on my shirt did cause some scabbing after three days or so. I had a leather coverd pad installed to solve this and it did.
I believe a srtaight gripped double rifle would be very quick and great for driven boar or deer hunting and was probably built with that in mind. I doubt recoil would be an issue in the calibres needed for that. For a DGR I think the pistol grip, even a pretty open one would be better by far. As for the 9.3 X 74, I've never shot one so don't know what the level of recoil would be.