Now if "research actually established that American models are much less safe when it comes to front-side collisions" then those differences presumably apply to US models that aren't currently sold in Europe, and thus have never been tested here. If a model eventually will be sold in Europe (in any numbers) it will presumably be tested in Europe too, and those safety differences would show up in the tests?
It doesn't worry me that some new car is approved for sale while not being safe, I wouldn't buy it until I see five Euro NCAP stars, and that I think goes for most European consumers. Relaxing the approval wouldn't change the testing! If US manufacturers want to sell cars in Europe they will have to not meet regulations but satisfy NCAP.
(That said: cars like the new mustang might never be tested in NCAP, nor will it have to to compete with European performance cars which often aren't tested either, or score poorly. Performance car buyers don't seem very safety concerned so crashing expensive cars that sell in small numbers doesn't seem worthwhile).