Since apparently the question "Are the Shoshone overpowered" seems to be revolving around the tall vs wide debate... I'd like to say that, from my experiences, I found it far easier to effectively manage my gold and happiness when wide (Shoshone, Poland) than when tall. And even culture fared quite well.
I also used to think, like many other people, that with BNW, going wide, rexing or ICS had been considerably nerved (science penalty, no more gold from rivers...).
But on the other hand, wide empires are better for a religious playstyle. You can amass faith from shrines and temples, which with the adequate Piety policy can be built quite fast, even in small cities. Tall empires that want to follow the religious way have to rely more on what is unsure : getting Stonehenge, or the right pantheon belief to extract faith from terrain...
And once you have your religion, its very easy to get happiness and culture from pagodas or mosques (preferably both, but having both is a luxury).
Then, the rebuffed Piety comes into play. Now Piety is much stronger, so its perfect for that "wide and religious" strategy. (especially if you can get Jesuit Education)
Its true that my wide BNW experiments were weak in science at the beginning of the game. But they proved later incredibly strong, generating much more faith, much earlier, than my tall empires.
So... here come the Shoshones. If you time well your ruins bonus, you can get a lot of faith from scouting. I personally got 20 faith at first, and later 60 ! You will almost certainly get your Great Prophet very early.
And while you are still weak in science, your defensive bonus from the Shoshone UA can prove very useful. If the AI even bothers to declare war, which it doesn't seem to do any more (with the exception of warmongers like Shaka or Genghis).
Moreover, wide empires produce more more culture, and for this reason resist Tourism better.
To sum it up... Are the Shoshone overpowered ? I don't know. But they're certainly very strong.
And I forgot : the American Manifest Destiny discount on tiles looks a bit... meh in front of the Shoshone huge cities. Well, Manifest Destiny is more than that but still. The Shoshone are IMO better at throttling an opponent by claiming all the good land on the map.