Certainly a seemingly fun senario but in my mind, the 'reality' would not be pretty.
In some part, history portrays Buffalo Bill as an Indian fighter, and as well, a fellow willing to push himself to the extreme.
I 'think' Mr. Cody was likely an Alpha male and having a TW during his time would have afforded him the opportunity to "kick some serious ass".
I suspect this was not what you were thinking of. My thought is, during 'that' period, folks did not hit the trails for fun; it was pretty much life, death or financial gain.
Look at what the repeating rifle did for the development of the West. Sure a hunter could kill more deer (faster) to feed his family. I suspect however, deer were not the only things to die as a result of the temporary imbalance in technology. The TW would have represented a huge power advantage to anyone that owned it. I would think it unlikely that anyone would have thought neat, 'now I can explore more trails'.
Without question, there would have been alot more area to explore, but when it comes to that level of exploration, my thought is, the horse would be hard to beat. Now when it comes to robbing a bank and making a fast get-away.............. Just my thought. Gerry