Robert,

 

I think the Lancair Avionics mounting method is more structural that to provide a ground plane.  I recall an article in one of my magazines that tested GPS antennas and ground planes.  I'm going to dig out my box of old mags and see if I can find it.

 

On another note, my Garmin portable has a small 1 1/2" square antenna and it picks up satellites very well, no ground plane.

 

Mike