I don't know authenticity from an honest politician. But here's my experience with the GPR:
I started out with a factory model in 54, then picked up a kit in 50. There's extra wood on the kit, so first I dragged it back down to factory dimensions. Due to the extra metal in the barrel on a 50 rather than a 54, I wasn't as happy with the balance of the 50 compared to the 54.
So I went to whittling. I slimmed the fore end, flattened the cheek piece a bunch, got rid of the perch belly and generally removed wood anywhere I could. Ended up taking a full half pound of wood off by the time I was through and got the balance I wanted to boot.
The end results are a lot slimmer than the factory GPR, along the lines of a "Kentucky" or "Pennsylvania" or some such. It really is a bunch more pleasing to my eye and for my handling, whether or not it looks like a "real" plains rifle. I don't really care what others think, as a matter of fact. It's my gun for my shooting.
I was really surprised in the recent thread about shooting an original Hawken. The rifle was a whole lot trimmer than any modern repro I've seen. Kinda makes me want to compare it with other originals to see if it's unique in that regard, or if in fact the other originals are trim too. From a pure handling standpoint, I have to believe the first purchasers would have preferred trim to clunky, but what do I know?
Gotta say though, that my factory GPR's are in real danger of visits from the wood rasp. There's a lot more wood than needs to be there for anything but "style," and trimming sure turns them into a better overall gun for me.