I added a low speed (600 rpm) 140 mm side fan to my Fractal arc midi and my Mobo temp dropped 3 degrees. Sadly, did not record GPU temps.
I have 4 induction fans and 3 exhaust fans on my rig , all the latter in the top of the case, 2 on they hydro 100 , and one in top back. I was surprised to get the result I did , especially as I already have an induction fan in the very bottom of the case slightly further forward than my side fan.
Fans are quiet, the side fan was a fractal arc, recomended , very quiet.
This is a good site for fan speccing http://www.quietpc.com/gb-en-gbp/products