Ritz ran 1:00:12 behind Farah just a few months back. We have a good enough development system and plenty of talent to not take third-rate Kenyans who didn't move here until adulthood to fill our teams, though Chelimo has certainly proved on the track that he could make the leap to the first tier of Kenyans. But it's obvious that those new Americans are doing about as much as well or better as Tiernan, so talking about them wouldn't exactly answer the question.
As for the heat/cold:
in Poland in 2013 in snow and ice, True took sixth, Birmingham eighth, Derrick tenth, Ryan Vail 16th, and Mack 19th, plus the Aussies Adams and Robinson were also top 30 and the American Elliott Heath took 30th (American James Strang was 37th); and the U.S. team with no Kenyan imports took silver by two points over Kenya!
Kenya and Ethiopian highlands are obviously not superhot a lot of the year but the fact is that Kenyans don't dominate as much in cold conditions. In 2001, in cool rainy, they did win but Ethiopia was ninth, while France was 2nd and the U.S. 3rd, while in the junior race Teg was fifth and Ritz third.