Don't be silly. The Heathrow - US market is incredibly competitve market, and stopping off in Shannon is not competitive and it's still quicker to join the queue for immigration in the US !
Define competitive as if Irish Govt decides to charge minimum tax on transatlantic flights then adding in extra 90 minutes on flight and saving money PLUS immigration and customs time can make a huge difference both ways.
Same flight from SNN & LHR to JFK in Jan and return Feb hits £338 with BA and £304 with EI.. Get rid of the TAX from SNN and starts to become very competitive.
Add in through ticketing with Jet Blue.