Tom,
First of all, congratulations on getting it to shoot that tight. If you expect to shoot out to 100 yards I suggest that you are going to need good sights on the gun, you can adjust the POI using the sights. As to why it is grouping left, it could be as simple as how the barrels are fitted together or maybe you are putting too much of your trigger finger into the trigger guard and pulling the trigger off a bit. If your gun shoots mostly to the left, chances are you are using too much of your trigger finger, to the right, it is the opposite.
When I shoot for regulation I always shoot over a good solid bench because I want to know what the gun is doing, and not how well I can shoot it offhand. Next I concentrate on pulling the trigger with the pad on my trigger finger and pulling it straight back towards the junction of my hand and thumb, this makes sure you are not pulling it off one way or the other, and it is easy to do and hard to self analyze the condition causing the problem.
Another thing that I would mention to you is that those slug loads are really hot and may be effecting the way the gun shoots. I don't know what kind of country you hunt in but if it is woods hunting, you will hardly ever get a 100 yard shot in the timber. I strive for 50 yard slug gun accuracy and call it good.
Lastly, if you don't have a good rifle type rib on the gun to accept solid iron sights, I can help you out with that. You need good sights on it to make it repeatable. You have already got it shooting tighter groups than many guns that had been made up recently, keep at it and you will get what you want. You also need to realize that your barrels are crossing at 25 yards, that means at 50 yards the shots will be apart about the same distance as the barrels on you gun at the muzzles. This is still very good regulation of a slug gun. Bob