Another thing is in most of my designs I don't have power planes, I use mixed signal/power layers and lots of GND planes. If you have lots of gigabit signals, then I would recommend this approach to you. I have power planes on 14+ layer PCBs, between the out-most GND plane and the outer (top or bottom) layer. Also, the top/bottom layers don't have any or many impedance controlled tracks, only fanout. Usually short fanout tracks are not impedance controlled anyway.
Marce, I have stackup constraint that does not allow me to have true stripline, between ground and mixed power planes, how should I route my critical signals after fanout?Or you can have specific layers for high speed signals that are true stripline (between two grounds) and run other non-critical slower signals on other layers, where the signal coupling isn't as critical.