I have an original AMT '67 Cougar kit on the bench. I have robbed some parts from an AMT '69 Cougar XR-7 kit issued in 1977. The '67 body is solid as a rock. Very sturdy. Easy to work with, as I hacked off the top with a razor saw, having no problems. The body in the '69 kit is so badly warped, I won't be able to use it. When I pick it up from the box, it wants to try to just fold up on me.
So, that is a difference in styrene from '67 to '77. I, too, have noticed a difference in the styrene through the years, as well as a difference in styrene from one manufacturer to the other. They don't all seem to use the same formula.