Would speeds of that kind be safe for a majority of transatlantic aircraft
If it wasn't safe, they wouldn't fly it.
IIRC, its relative airflow that matters in relation to airframe integrity, which is why airframe limits are specified on an Indicated Airspeed basis rather than True,Ground or Calibrated Airspeed.
If you think about throwing a tennis ball inside a moving car, the ball will move at speed X but combined with the underlying speed of the car it will have moved at the vector sum of the two speeds and thus covered a larger distance in a shorter amount of time than it would have ordinarily managed to do at its original speed.
In much the same way that if you put a tennis ball infront of a hairdryer pointed vertically upwards, the tennis ball will settle at a fixed location close to the hairdryer... it won't go rushing off into the distance.
At least that's the way I understand it, no doubt in true PPRuNe style, someone vastly overqualified who got a 1st in Aeronautics at University will be along shortly to give you the fully technical explanation with fancy formulas included .... in the mean time, here's a video of a man in a white coat playing with his balls....
As it happens, I flew the Miami route with a 200kt tailwind fairly recently and it seems I survived to tell the tale, as did the aircraft....the only bad part of the whole flight was having to hold until the Heathrow NIMBY 6am limit passed so we could land ...
