Adding an extra fan should lower the temperature as well as increasing the noise and cost.
Better to just run the provided fan at higher speed since the increase in noise is much lower than could be expected...
... speaking of which:
Exactly how is the noise measured?
It only says that it's done by a mediocre audiometer (+-2dB or so at relevant levels) and the result is "adjusted to 1m".
* Where is it measured?
* How is it "adjusted"?
* ... "to one meter" in what direction?
* What is the background noise level?
Also the "Noise efficiency" formula is totally flawed:
By the formula 50C dT @ 50 dB is just as good as 10C dT @ 10dB, which it obviously isn't!
A step in the right direction would be to use the difference in cooling and noise between fans at half speed vs full speed.
( (dT(50%) - dT(100%) ) / ( Noise(100%) - Noise(50%) )
That way you see some actual efficiency in terms of noise cost to improve cooling.
To make it even better the noise value should be made linear in the formula.
Linear noise = 10^([dB]/10)
None of those changes make any sense unless the measured noise values are reliable in the first place though...