[SI-LIST] 8-layer PCB Stackup - Follow Up

I like stackup A, but have the following question.

If I make the spacings between layers 3 & 4 and between layers 5 & 6 big, 
this will affect the trace impedance on layer 3 and layer 6 respectively.  
Therefore, I'm thinking about keeping these spacings small, but increasing 
the spacing between layers 4 and 5 (between +5V and +12 planes) to meet the 
required PCB thickness.  Are there any disadvantages?


1) Signals
small space
2) GND
small space
3) Signals
small space
4) +5V

BIG space

5) +12V
small space
6) Signals
small space
7) GND
small space
8) Signals

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