I have used bore butter in the past with no bad results and it makes a good patch lube for PRB loads or bullet lube for conicals, but if you clean the barrel thoroughly after each shooting session I just don't buy the "seasoning" theory. If you thoroughly clean the barrel there is not going to be any bore butter left, and if you aren't getting it clean you will eventually have rust!
I use a good cleaning, followed by acomplete drying, followed by a good gun oil for rust prevention these days and get good results. Before shooting again, clean out the gun oil with alcohol soaked patches followed by dry patches (or just soak up any excess with dry patches on some barrels - each barrel will have it's own "attitude" toward what it likes and will tell you by where the first shot goes, some even demand a fouling shot).
The poor groups could be from a lot of variables. Powder type, granulation, and quantity - bullet type and weight - loading procedure - or just a poor barrel or bedding problem.
Usually about any muzzleloader can be made to shoot well with the right load combination, but there is the occasional "lemon" in the bunch. Over the years I've found a few that shot about anything you loaded in them well, but more often they prefer a powder charge in a small range (say 80 to 110 gr for instance), and a general weight range & bullet style (short or long, light or heavy).
I usually look for a bullet type & weight that gives better results in comparison to the rest, then play with powder charges - but you can reverse the procedure and figure it out, too. I would recommend varying only one component at a time when looking for that "sweet" load.
Not sure of the max loads for that rifle, so I can't advise you on that issue.