You may want to change the fan speeds as well. I'm not sure yet what the optimum is...
I did a little experiment with SP30s. I took 10 machines, and gave the even-numbered ones a fan speed setting of 80 to start with, and the odd numbered ones a fan speed of 70. I then waited an hour, and recorded their temperatures and performance (from monitor.php). I then switched them up (odd 80, even 70), waited another hour, and recorded again. Here's the result:
jtoomim@server:~/wissamperf$ python ~/fan_comparison.py
==== phase1 ====
machine-name timestamp hashrate front top bottom is_odd
wissamitter-10 1415393491 4601120 17 70 68 0
wissamitter-1 1415393491 4593400 17 78 75 1
wissamitter-2 1415393491 4585680 16 71 69 0
wissamitter-3 1415393491 4582785 17 79 74 1
wissamitter-4 1415393491 4560590 17 72 68 0
wissamitter-5 1415393491 4576995 17 75 73 1
wissamitter-6 1415393491 4596295 16 70 70 0
wissamitter-7 1415393491 4574100 17 73 71 1
wissamitter-8 1415393491 4562520 17 69 67 0
wissamitter-9 1415393491 4606910 17 78 74 1
==== phase2 ====
machine-name timestamp hashrate front top bottom is_odd
wissamitter-10 1415396747 4591470 17 73 72 0
wissamitter-1 1415396747 4585680 16 73 69 1
wissamitter-2 1415396747 4533570 16 74 73 0
wissamitter-3 1415396747 4576030 16 72 68 1
wissamitter-4 1415396747 4558660 16 74 71 0
wissamitter-5 1415396747 4571205 16 68 66 1
wissamitter-6 1415396747 4596295 16 74 73 0
wissamitter-7 1415396747 4562520 17 68 68 1
wissamitter-8 1415396747 4568310 17 72 71 0
wissamitter-9 1415396747 4581820 16 72 69 1
==== Summaries ====
=== Evens ===
== phase1 ==
{'count': 5, 'front': 16.6, 'rate': 4581.241, 'top': 70.4, 'bottom': 68.4}
== phase2 ==
{'count': 5, 'front': 16.4, 'rate': 4569.661, 'top': 73.4, 'bottom': 72.0}
=== Odds ===
== phase1 ==
{'count': 5, 'front': 17.0, 'rate': 4586.838, 'top': 76.6, 'bottom': 73.4}
== phase2 ==
{'count': 5, 'front': 16.2, 'rate': 4575.451, 'top': 70.6, 'bottom': 68.0}
== High speed ==
{'count': 10, 'front': 16.4, 'rate': 4578.346, 'top': 70.5, 'bottom': 68.2}
== Low speed ==
{'count': 10, 'front': 16.7, 'rate': 4578.2495, 'top': 75.0, 'bottom': 72.7}
That's 4578.346 GH/s at 80, and 4578.2495 GH/s at 70.
Conclusion: if your intake temps are around 16C, and you're using September SP30s, fan speeds around 70 or 80 seem to be equally good.
Source code available upon request.
