As near as I can tell, very little of the air pushed by the Noxania fans against the stack of 3.5" drives actually penetrates into them. When I put my fingers on the side of the drives that's opposite the fans, I'm feeling very little if any airflow. The drives are packed pretty tight, so it's notionally similar to aiming a fan at a brick wall. Most of the air moved by the drive fans seems to be blasted backward toward the fans or travel sideways. So, I can believe the side of the 3.5" drives that is facing the fans might be getting some cooling, but the rest of a drive isn't getting as much help.
At what location on the hard drive does Western Digital put the temperature sensor? If it isn't on the hottest part of the drive, then there's going to be a temperature gradient between where the sensor is measuring the temperature and what the hottest temperature is. Which temperature is the one that matters? I'm guessing it's the hottest temperature. When you pack a bunch of drives closely together, and even more so when the cooling is asymmetric, that's going to change the temperature gradient. Most likely the hottest parts will get even hotter, especially relative to where the temperature sensor is. If the temperature sensor is closer to the periphery of the drive, it becomes a less good proxy.
So, after thinking about it, I'm realizing I don't know enough to confidently evaluate the HDD temperature data, if I were to collect it, unless the temps reported were obviously low on the face of it, as they appear to be in chinesestunna's rig. There may be a lot of implicit assumtpions baked into a seemingly simple "operating temperature" number.
Given all that, I'm inclined to err on the side of more cooling ventilation than what's needed rather than risk not enough cooling ventilation. Maybe that's just me, though, and someone else, with more knowledge about the background assumptions, might well reach a different conclusion.