Here's my opinion of what America should get for their UA, as I do think it needs a boost:
I think America should keep their current UA, but also get a, say -10% to -20% cost to upgrade obsolete military units. The exact percentage is flexible.
My reason?
With the exception of very recent history, America never entered a war with the best military technology but almost always ended the war with it. I.e., Union Forces in civil war, WWI, WWII, Korea, Vietnam, etc.
Unlike virtually all the western / asian militaries, America has never stubbornly clung to obsolete technology for their military out of vanities of honor, tradition, etc. etc.
OR another option would be:
Cities with or (without ?) a factory gain a +10% to 25% production increase in military or naval units DURING warfare with a another Civ (not a city state).
A historian once wrote, that America's greatest contribution to WWII was a factory in Michigan that produced a fully operational B17 every hour, 24 hours a day. In brief, once mobilized for war, American economy can produce military machines faster than any army on the planet can destroy them.
I know this sounds like a "rah-rah- go-America", but I believe that either of these two UA's would not be too OP'd and more in line with their historical strengths than +1 sight or tile buying.