I, for one, can't even guess Tom. Up until I made your rifle, I had never even opened a can of Triple 7.
But, with Black, trying 2, 3 or even 4 wads can sometime make a difference. Now 2.25" at 100 yds with a muzzleloader is VERY god, but you are correct in assuming it can get better. I would have to put the practical limit at about half that, because I have never heard of anyone that could do much betted than 1 1/4 with any iron sighted muzzleloader on demend. Sure, you get one now and then that will go less. I once shot four 54 caliber round balls into a group of .825" center to center with a FLINTLOCK --------- but note how i remember it. I have never done it again.

I can get 2" with most of the rifles I make, but under that is getting to be harder and rarer for me. I think my rifles may do better, but I can't do much better now. I am not young anymore, and my eyes are starting to give me a bit of blur now.
But, if you want to play with things, the first thing I would try is a thicker and softer lubed wad. The felt I sent with the gun was as thick as I could find around here at the time, but I do find it now and then up to 1/2" thick. I am sure the Eastern Seaboard has more places to get things like that then Wyoming does.

Let us know how you do.
happy hunting
Steve