So I think you are going to be stuck with higher RPM fan's unless you underclock it. And I would monitor the heck out of it if your messing with fan's. They really were not made to be quiet miner's. There are some good mod's people have done on forum to quiet them some though. Going to S3 level fan... I don't think is one I would do.
If I backed off freq to ~400, and put a S3 type fan on my Batch 6 S7.... I think it would still overheat on a day with ambient > 85*F.... But why do that when you can just turn it down to 30% or less fan speed with the stock fans. I would never consider S3 fans on an S7... The S7 needs the CFM.Do two things:
Create a cabinet that is insulated to house the miner and its power supply. Have a ducted inlet to the front, and have the exhaust ducted directly from the fan to an exhausting source. the further away, the quieter it is. you really only need like 6-8Ft if you are ducting directly to a duct vent.
You only quoted half of my comment. The bold part I do not disagree with it is entire reason for part of my post you did not quote. S3 and S7 are massively different miner's. Just look at watt's S3 366ish and S7 depending on batch 1293ish . So were talking like three times the amount of watt's which means more heat. :
I'm don't think S3's and S7's are even in the same ballpark. If your looking at a fan that was mentioned great for S3..... chances are it is not enough for S7. Specifically for those of us in summer. I did have one of the S7-F1 variants which had 1 larger fan it actually worked with out issue, but was a higher RPM fan.
Yes you can do things to make it quieter like you mentioned. But before doing all of this I would do some ROI math as S7 + building a decent cabinet to dampen heat that does not heat up crazy will not make sense for a lot of people. All depends on electricity.