With the SAA grip the pinky is supposed to sit under the butt for lack of anywhere for it to rest on the front of the grips. When I started out this felt odd so I choked up on the gun as Mark II describes above. But when I did that the gun shot strongly to the left and I had to twist my hand way around the grips and into a pretty unnatural hold to correct that. Then I moved my hand back down so the web rested on the turn of the back strap and my pinky sat neatly under the butt of the stocks. This also fixed my reach to the trigger and made the gun feel far more comfy in my fairly large hand. And once I adopted this method the POI matched the POA without any need to twist my hands around to any sort of unnatural grip.
What the pinky does when it's down there is index the gun after it rolls within your grip when shooting strong loads. Or if you're shooting light mouse fart loads it at least let's you know that it's in the right position.
Now for the bad news. For strong loads and for those of us with larger size hands the classic size of stock grips tends to be somewhat small around the neck just behind the trigger guard. So the hot loads tend to rotate the gun too much in our grip and in some cases (google for "dragoon bite") the gun leaves the back of our saluting finger battered and bruised.
The fix for me was a set of fatter necked custom grip scales that I made. Now I can shoot the hottest .44Mag loads without fear of dinging my saluting finger. And this is despite shooting these loads one handed.
Sorry but I can't help out on the Bisley style. I've never been given the chance to hold one let alone shoot such a gun with that style grip.
As for the longer barrel there would be some gain in velocity. Also the longer sight baseline would show you the slightest error in the sight picture. But I've found that even trying to hold a 7.5" barrel to a steady sight picture is already hard enough. I don't need a longer barrel to make me think I've developed palsy or some other ailment....
If we were allowed to hand gun hunt up here the longer barrel would likely really shine for the extra 100fps or so it would provide as well as giving more barrel to support in a tree or bush notch while the shooter takes aim through a handgun scope. But we aren't allowed such things so for me I just think it is too much of a good thing. I find that a 7.5" is pretty much the best of the bunch.
Your unburned powder won't burn in a longer bore. Some powders are just like that. Something about the pressure wave spits some of it out before the flame front hits it or something like that. A longer bore would simply move the impact point of the particles a foot or so farther downrange.