I think that the engineers knew that the round port intake and exhaust were not all that much better than the regular ones. They made for great advertising though. The stock Pontiac four barrel intake is considered one of the best ever designed. The RA/HO exhaust whether D or round port made the biggest improvement. Later independent testing from people like the Hand brothers proved this. Maybe if you were trying to squeeze the ultimate last bit of power in a racing situation, the round ports were a small benefit. But to supply round port stuff all across the Pontiac engine family was probably not cost effective, nor a justifiable power benefit.

Remember we are talking stock engines in cars that had warranties. Also, at that time, perceived status was very big. Ram Air, cowl induction, hood scoops, all added up to advertising status symbols. Most of these had no actual benefit. Even the second gen TA hood scoop was located in a dead air space and not much benefit. They later closed it off so there goes any performance gain. The Ford and Olds models that used two of the headlight openings for ducting cool air into the air cleaner were much better performing.

Having used the long branch D port exhaust for many years, my seat-of-the-pants dyno sure felt like they made a huge improvement.