1) From your description, I get it that you do not plan to remove the shroud+fan of the display card.
If so, then I think it is not a case of diminishing return but it is a case of how you want to design the airflow path inside the case.
So, let me comment in general:-
The display card is likely to have a feature of zero rpm, allowing the card's fans to stop when the gpu is under, say, 50C. If you consider the card by itself alone, then this is fine. But I think you should look at the whole picture, ie the entire case airflow. I mean, the 2 case bottom fans bring in fresh ambient air to cool the card as well as supply air to other components (CPU, SSDs etc).
They are on all the time and the fan curve is adjusted according to the CPU temp (preferably). So, even when the card is not hot enough to turn on its own fans, the 2 case bottom fans will still be spinning to intake fresh air into the case actively. Additionally, these air will cool the display card, delaying the need for the gpu fans to come alive. This has the benefit that the 2 case bottom Noctua fans being more quiet than the two (or three) gpu fans.
