Started to get one of the Lee Shaver medium sights for the fast twist .40 GPR. However, the lowest setting on the sight would have been way high over the top of the barrel. The sights are made for shooting far with lots of drop. In the case of the GPR it would have required a huge front sight to shoot at a hundred. And yep, it broke my heart. Sorry - I got your quote included in the body of my post by accident.
OK...you do have to match the front and rear sights. If you are looking at a standard blade front sight through a Lee Shaver rear sight then you are probably getting way too much elevation. You should opt for a globe front sight as well as the rear tang sight or at least a very high front sights. I just looked at a photo of the GPR and can understand the problem - short tang means having to attach the sight over the center line of the rifle bore - but it can be done. If not with a Lee Shaver sight then perhaps a No. 2 Lyman peep sight coupled with a Lyman globe front. Either way, that is going to provide a better sight picture than a buckhorn sight with a blade front. I also note quite a few posts about maximum ranges of muzzle loaders, with which I tend to agree. Specialty rifles like the Gibbs and Whitworth, were designed for extreme (for their day) long range shooting. Plains rifles and PA long rifles were not. You can stretch range and accuracy just so far with guns of these designs and I have to wonder whether or not it is worth all that effort. In Scotland, for example, a place of considerable open space, hunters have always stalked their big game in an effort to get close enough to not miss. Maybe that is the best way to handle this situation too.