If right now you have two fans “screaming” at 8500 RPM AND your SFP’s are nonetheless at 71°, if you reduce the RPM’s the temperature will raise further.
Yep, but usually replacement Noctua fans with the same diameter are not more silent while delivering the same airflow, they are more silent while delivering a much lower airflow.
They are a great solution when the device has (relatively) low traffic and (relatively) low temperature BUT the original fans keep starting and stopping (and they are actually loud).
If you already have (relatively) high temperatures you need to increase cooling while reducing noise.
Depending on the SFP module you can sometimes use an external heatsink on the part of the SFP outside the socket, like:
http://forum.mikrotik.com/t/sfp-module-is-extremely-hot/117746/1
About noisy fans, the only viable solution (that I know of) is replacing the fans with larger size ones (that can deliver the same airflow at less RPM, thus with less noise) and “invent” some adapter, example:
http://forum.mikrotik.com/t/seeking-help-ccr1016-12g-router-fan-voltage-model-and-replacement-due-to-noise/173160/1