I'd be real careful drawing an assumption that mounting inside of the cabin cover works well. Not having tested the cabin cover for attenuation at 1 Gig and not knowing if mounting it inside will expose it to low angle attenuation. Further, considering that the antenna mounting instructions state that the antenna should not be painted, this because some paints contain metallic particles which can cause attenuation.
On the flip side, if you are just using the GPS for VFR flight, and willing to accept the consequences of possible a drop in signal, then it probably does not matter.
IMHO I would recommend always mounting a GPS antenna outside, on a location away from the tail, and on a surface that is as parallel to the fuselage flight attitude as possible. Since GPS antennas have their own ground planes, they do not need to be mounted on a metallic surface. Being in the RF electronics business for 35 years has taught me it's best to follow the manufactures instructions.
Bill