Well the Enermax fans I am looking at http://www.enermax.co.uk/tbsilence.html
are rated for 42CFM at 11dBA
Sorry for being so direct: not even by mistake
any 120mm fan on Earth may give you in any situation 42CFM at 11dbA, as physics laws cannot be overcome with so little efforts.
Please, don't trust those figures as specs, as they are likely marketing hypes, and because, to be fair, Enermax doesn't declare -, in their facts sheets - that it gives 42CFM AT
11dBA, but just it gives 42CFM AND (so, more probably that not, read as OR)
11dBA (likely the noise at the starting voltage, around 280rpm).
Facts sheets are often made this way, in order to suggest but not to state firmly.
Moreover the particular modular metal/plastic frame (that which is not solid it is prone to vibrate/resonate) and the brittle transparent plastic of blades - while allegedly they reduce turbulence noise - are likely to amplify vibrations, when the fan is screwed to a metal case and then to a filter (as in a GD05). On the contray it is very valuable that Enermax bearing should be much more long lasting.
Have you given a look to SPCR test of that Silverstone dimpled fan, haven't you?
and I do like the fact that they have detachable blades for cleaning, I'm a bit OCD about things like that.
If you aestethically like those Enermax, please do it, buy them: moreover they should be very cheap.
But if in case do it for the sake of modding, not for the sake of silence or cooling, as Silverstone fans are far more than adequate at any purposes and with an excellent tonal quality. Last but not least, they are a proven design, with measured figures, not declared ones.
Swapping them IME the best you might obtain is that almost nothing changes (sonically).
Just as a side note, more probably that not current Silverstone lineup offers the best (sonically) stock fans available, better than Fractal ones, and IMVHO better than any
shiny Enermax: moreover, even if the Enermax should be cheap, if you own a GD05 you've already paid for them.