Thx [USER=26]@Phuncz[/USER], your appreciation is quite right I think.
I could force more air to go through the fins using a duct, but it will need more room for it and actually I want the fan to cool other elements too.
As you say, I would like to use the more dense fins structure while still letting air going through it.
Let's say this heatsink is supposed to cool down a CPU of 100W, according to its size (12*15*2.5 cm), is there a way to calculate how many fins are needed?
On another hand, with the inner fan extruded heatsink, I will not need any more room for the fan which is a plus.
Question is, with the same volume, what kind of heatsink is supposed to work better?
I guess I either need samples to check that or get a physics teacher to calculate this! 
