I can't say what will work best in your rifle, but I can share some experience.
I shoot 45-70, PP and GG, with Black Powder, and exclusively use Olde Eynsford 1.5F. I compete with that rifle, but it's all 300 yards and less, so may not tell much. I use a lot of compression, much more than conventional, and have low extreme spreads and an SD of 11, which is good enough for me.
I recently got into a Mauserbuchsen, single shot, in 9.5x47R, .375" grooves. And I tested with OE 2 Ffg and 1.5 F. Despite the 360 bore, this rifle didn't want to shoot 2Ffg at all. It was remarkable how much better light compression 1.5F was with both PP and greasers. The difference between groups and patterns.
So, this sort of flies in the face of conventional wisdom that suggests 2Ffg starts to deliver improvements at .40 cal. If you were getting 2MOA groups with 1.5F, I see no reason to change. As to Swiss vs OE, I have used both and I am not a good enough shooter, or do not shoot at long enough range to see a clear advantage for Swiss, but folks that shoot better and longer than I do swear by Swiss.
In other words, clear as mud.