Thanks for your reply and explainations!
You are right, fractal noise works better as displacement map, and I see there is some intrinsic reason for it now :-P.

However, this does not explain why our output is darker, and I still think there is something wrong there; I noticed the turbulence alpha channel affects the output of the displacement mapeven when the latter takes values in the color channels only:

Do the following test:

Create a filter with
- turbulence primitive
-color matrix
-displacement map

Set the color matrix to do nothing (only 1s on the diagonal, 0s everywhere else and in the last column).
Set the channels in displacement map to Red and Green for instance, give scale some positive value.
Observe the result.

Kill the alpha channel in the color matrix, i.e. change the last coef in the last column of color matrix to 1:

See how the output is modified, while the displacement map is supposed to take values only in the Red and Green channels (and not in the Alpha channel).

I suspect we are using premultiplied alpha where normal colors should be used, but I remember I already bumped into similar strongly non intuitive conventions of svg with this respect (maybe the same one, I don't remember), so I'm not sure.

cheers, jfb.