It's been done. Why do you want to run a spacer? I don't see the point.
I do not have "hard" data, but logic and use seems to show better air flow on a modified GN.
If you visualize the plenum as a "reservoir" for air, the greater volume is more able to supply air to a given cylinder when the intake valve is opened.
With 9 sec. and faster cars, we can run less boost since there is more air available and the IAT is less with not as much restriction.
I have seen GN's with a Hemco or Precision plenum pick up a little HP because the cooler intake air will allow more boost.