Why do you want to lower the hashrate? There are no efficiency gains that way, same ~0.55W per GH/s or whatever it is.
Three reasons:
1) Heat is a real problem and I need to reasonably minimize it. The S3 tends to stay under 50C all year 'round, but that's clearly not the case for the S5, which apparently easily lifts above 70. Come summer, I can always paddle around in my underwear, but that doesn't help with cooling the S5.
2) Noise is a real problem and I need to reasonably minimize it. The S3 push/pull fans help tremendously, but they still sound
much more earnest than the S3's sharing the same room. (Near 2:1 fan speed and over 10C difference.)
3) According to "BitmainWarranty", they experienced reliability issues after a few weeks of running the S5 with the Corsair CX750M. I want to ease the load and re-use the PSUs I already own (previously powered the now displaced S3s.)
CoolMaster 1000W PSU will be 1 x S5. 2 will die or see lots of lovely Xs
Also, Corsair cx750m will work for now, but may experience high failure rate about 3-4 weeks into it or later, and HX850 is recommended.
S5 is out in the market for only 2 weeks. So, you may not experience much failure yet but we have used CX750 in heavy miner deployments in the past and seen what it does. (360w-400W sure, but not when it is pulling 590-600W constant)
PSU, please remember, you will need minimum 2 rails of 12v with TICK GAUGE PCI-E cables or else, it will get hot and may melt or may catch fire.