In Mega Taiga there normally are these SurfaceAndGroundControl settings:
DIRT:2, DIRT, -0.95, DIRT:1, DIRT, 1.75

Is there a way to make it like in WorldEdit, where you can add more than one material and give them percentages like "75%Dirt;2;25%Dirt:1, Dirt, -0.95, [...]" ?
If not, can you add it or is it impossible?

As a quite circumstantial solution i would try to list a lot of Noise Values like:

DIRT:2, DIRT, -6.5, DIRT:1, DIRT, -6.0, DIRT:2, DIRT, -5.5, and so on...

but i guess it consumes a lot of power and makes the generation a lot slower, right?

So... Apperantly it doesn't cost much power, world generation is maybe 1 second slower than before, but i can live with it :D