Possible to switch to musket cap nipple on GPR? Yep. Been there, done that. I determined that my experiement with musket caps was a solution to a non-problem and promptly went back to the original nipple. I have some kind of aftermarket Number 11 nipple on the rifle now, but I can't remember which one it is. Whatever it is, it works just as well as the one that came on the rifle when new, and that one gave no reason for complaint.
Is it worth the effort? Well, there really wasn't much effort involved, but even so, FOR ME, it wasn't.
Worth the expense? Again, it wasn't for me.
I've had my GPR so long now that I can't remember when I bought it. It was a sweet shooter from new and remains so now. Ignition has been very reliable, provided that I've really made sure that the bore (breech in particular) and snail are clean and dry (as in de-oiled) prior to loading. I've had a few FTF's with the rifle, but never when I've done a proper job of preparing it for loading in the first place.
JP