When you get to this level of gun the "judge the gun not the name" is as important as ever. Jerry Fisher does extremely high quality work, as good or better than the original maker. His restoring a gun should not detract from the guns' value.
British guns have always been back to a smith for a complete going over and we accept this as normal maintanance with no decrease in value. The new site may have a minor impact on value because it is different than the original but weither it is a plus or a minus is open for debate. The quality is there and the form and function fits the gun well. Take that gun out into the bush for a few years and you would be hard pressed to not view it as an original.
This gun is a peice of art as well as a fine tool. Just wish I had one in my collection. For that matter I would need a new sholder and back to shoot it well. Hard to get old after a fun youth.