SkunkApe,
While I agree wholeheartedly with the gist of your post, my understanding of the development of the SxS is somewhat different.
"For hunting fast-moving gamebirds in heavy cover, where snap-shooting is common, the side-by-side is the pinnacle of firearms evalution."
Wasn't the SxS game gun developed for aristocracy shooting driven birds in relatively open cover, rather than for snap shooting in heavy cover?
My understanding of the English shooting technique is that there is no "sighting" along a sighting plane at all. Rather, the eyes find the bird, the leading hand begins moving toward the bird as the gun is started towards the shoulder, as the gun touches the shoulder, the shot is triggered, as the leading hand has already placed the barrels in proper position. Sighting along the barrels just isn't done in that technique.
"Remember, a proper game gun should have double triggers and an English (straight) stock to allow thetrigger hand to slide slightly rearwrds to fire the second trigger."
Again, I agree wholeheartedly with how the game gun should be configured. However, my understanding of the purpose of the straight stoke is somewhat different. Consistent with the English technique, the forward hand plays the major role in shooting, and the straight stock puts the rear (dominant) hand in an awkward poition - it somewhat "kinks" the hand/wrist. This causes/allows the forward hand to take over, as necessary. I have triggered a fair number of rounds through straight stocked, double triggered guns, and I can't say as how I have ever had my hand slide to rear to engage the second trigger.