There is no diminishing return or max AP. AP are incredibly useful in navigating recruiting.
From the dev Q&A a while back - "It takes roughly 50 AP to build the same amount of interest per cycle as a Home Visit.".. Think of 2-2.5 HV as 1CV.
My understanding is... AP, HV, CV, offered scholarship, and promises all are forms of 'effort' put towards a recruit. That effort is put into an equation that includes preferences and your current prestige. That spits out an outcome which, that final value number, is then put towards the recruit. The totality of all your final numbers get added up and put up against everyone else contesting that recruit.
In other words, pulling AP on a recruit you don't have locked up is a really risky play as the other team could just be pumping in AP and potentially knocking you to high, or worse, moderate where you don't get a roll.