I fail to see what that opportunity cost has to do with science per turn, to me it looks like a function of the tech tree layout and being able to choose the techs first and foremost. If you couldn't reliably go three techs down at some points with free techs but had to research it that would go a long way. It's just that being able to grab those techs immediately that should take you 40 or 50 turns to research is very, very strong because it puts you at a position in the game where you either get a lot larger bonuses from the city states, have a vastly superior unit type to everyone else, win the game, or any combination of the three. Unfortunately, what adds to the small opportunity cost is the low production rate compared to teching and the generally marginal buildings. In Civ4 I often felt bad about having to take a military tech because there were juicy economic techs (changing policies, adding yield, and so on) scattered all over the tech tree. In Civ5, there are very few of these. The only ones that have a significant effect are Civil Service and whatever buffs your lumbermills and even Civil Service is kind of moot due to the huge amount of food Maritime CS offer you for some money. I know that in the late game Biology will allow your cities to again grow in 5 turns but by that time the game is usually decided (in that either you or a run-away AI has a huge tech lead). The only times when I saw this 75% storage really kicking in was when I deliberately slowed myself to see some huge cities once.