Thanks for the info, it is very interesting, I did the exact same flight about 5 years ago, by the time I cleared the long queue of immigration and then the customs (which picked my luggage for inspection due to the damn fishing rod - once he saw it he laughed and waved me through...!!!?), I missed my BA flight - the check-in counter was closed. Luckily there was another flight to London a few hours later so it wasn't too inconvenient. However, I said never again and now always go via mainland Europe.
US will do themselves a huge favour and a lot more money/business for their airports if they could tighten their transit halls and luggage security and introduce full transit concept like others (no immigration or customs on transit flights) !