"If you have a multilayer board you get a distance of about 100µm between the individual layers."

Ehm, I don't think many of us actually design 16-layer boards on a regular basis, Kai ;o)
I do most of mine on 4 layers:

TOP ________________
GND ________________
PWR ________________
BOT ________________

which gives me 0.4mm spacing between the top and GND layer and 1.2mm between the bottom and GND layer for a standard 1.6mm board. As my designs tend to be 90% SMD, having the GND layer directly underneath the component layer gives the best performance in terms of minimizing inductivity.

An added benefit of a multilayer board with dedicated, solid power and GND layers is that the capacitance between these layers gives you a free decoupling capacitor! This capacitor may not be very large (several 100 pF), but it is of very high quality, remaining capacitive well into the GHz region.


"... it's of benefit also to have a bit ground plane between the traces."

I feel it's important to stress that any 'copper fill' between traces MUST be connected to ground!
Any 'floating' copper between traces spells trouble, if not disaster, because it greatly increases capacitive coupling between those traces. If your PCB design software does not have the capacity to 'shoot holes' (i.e. add via's at will), do not use copper fill.

Best regards,
Rob.