I also gave a Noctua 140mm fan a go.

I figured, this model "Noctua IndustrialPPC NF-A14" has a maximum effect of 156,5 CFM, which is significantly more compared to its 120mm model (109,9 CFM) as well as 2 dBA quieter.
This model http://www.amazon.co.uk/computers/dp/B00KESS5L4/ref=sr_1_7?ie=UTF8&qid=1439794766&sr=8-7&keywords=noctua+140mm

At the same time, to be able to fit it on the S5, I got a 140mm to 120mm adapter.
This model http://www.amazon.co.uk/Bitspower-BP-FA140120-BK-140mm-120mm-Adapter/dp/B003U3RTEO/ref=sr_1_10?ie=UTF8&qid=1439795104&sr=8-10&keywords=140mm+120mm

When it all arrived, I was expecting magic to happen, I was sure that I cracked the "code". No more fiddling with fan speed needed, and no more unnecessary noise. But that was short lived, as soon as I put it all together and started up the S5, it all came crumbling down, and I was back to square one. The fan did not perform nearly as good I as expected, at 26 degrees room temperature, it was running at max speed and keeping the miner at a constant temperature of 60 degrees. The noise at max speed (41.3 dBA), even though supposedly quieter than its 120mm brother, was considerable and once again louder than the original fan running at 35%. So yeah, one more bummed out experiment to nag about.

My next planned experiment, 2 x Cooler Master JetFlo 120mm max effect 95CFM - 28 dBA.
But this time, I am not as optimistic.