I really like these fans, I have 4xAF140's and the 2xAF120 performance. Performance wise the 120's are great, the 140's aren't as good if you compare the size of the fans, they move about the same amount of air, just the 140's do it much more quietly.
I also have that 200mm Bitfenix Spectre Pro, while it's got good airflow it seems to have nearly no static pressure and the motor noise seems quite obvious to me. I will be taking it out of my rig in the future to replace with 2 more AF 140's.
At 5v you will struggle to hear them depending on your case. At 7v you will most likely just hear the airflow and usual case vibrations hum. 5V is the lowest I can get them on my controller, they don't move much air at all at 5v though. 6.5V is the lowest I run mine at and the sound level is good.
7V will be fine, especially in the R4, could probably get away with 9V tbh and still not mind the noise, while having good airflow. Not amazing airflow, but certainly enough. At 12V my case gives me chills when I walk past it.