What should a 120 exhaust fan speed be like to effectivelly exhaust the air out of a solo case (for example) if your cpu would be cooled by a prolima genesis with both 140 fans on???
I have a Prolimatech Genesis with 2x Prolimatech Vortex 140mm fans on the CPU (using an i7 3770K at stock), and another Vortex 140mm at the rear in a Fractal Design Define R4 case. Potentially you don't really need to run the case exhaust at all (though I wouldn't advise this), as the back facing CPU fan is already somewhat close to the back of the case. (I've had the system on for a short while with only CPU fans at 500rpm, temperatures stayed consistent with what I normally expect at these speeds).
Never the less, my system temperatures are perfectly fine running all fans at 500rpm (the lowest you can basically go). Right now I've got the front fan running at 500rpm, a side fan running at 500rpm, one of the CPU fans running at 600rpm, the other at 700rpm, and the rear exhaust at 500rpm. My CPU temperatures are a bit below 40 degrees at idle (although I do have a fair amount of stuff open). On load the highest I tend to get is 50 degrees. For contrast, with all fans at 1100rpm in a non quiet case with lots of holes in the top I got 30 degrees at idle.
It's worth noting however that in my scenario, I have a passively cooled RAID controller directly under the CPU heatsink, and below that is the back heat spreader of the massive ASUS GTX 670 DirectCU II graphics card. The flow of the air from the CPU fans greatly helps to cool the RAID card. As such, I run the CPU fans at a slightly higher speed than I might otherwise desire in order to cool the card.
For optimum cooling, it may be worth investing in a fan controller (I have the BitFenix Recon) which can dynamically adjust the speed of the fans depending on the temperature at certain points in the case (utilizing the special heat sensitive cables that may be included).