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.

I bought a ton of fans to compare with and my experience is like most - I order them hoping for rainbows and unicorns, only to watch the temperatures rise to basically unusable levels.  The only real alternative I've found is this, the Ultra Kaze:

http://www.amazon.com/gp/product/B001JKNMBE?tag=ana024-20

It's not a silent fan, but it's much quieter than the stock fan... Although I will say that with the new firmware, set to manual speed about 30%, then it's much closer (I still prefer the Kaze).  The big downside with the Kaze is that it's a 3-pin connector, so no PWM control - but the upside is that it's cheap, especially compared to some of the other fans out there.

I did a bunch of tests of different fan configurations, and they generally don't make as much difference as you'd think - if anything, they do more to alter the temp sensor on the board than anything else.  My top 3 biggest improvements (in order of impact) are; fully enclose w/ fan spacer, kaze fan (pull), headsinks on all copper pads.  I've done it on almost all of my S5's, and in general I see a temp drop of about 10c over stock.