I have a Comet CA-2x4SR 2m/70cm mobile antenna on a trunk lid mount.
Comet tells me this is a "ground independent" antenna, so I've been
thinking about adding a short mast to get it above the cars roof line.
Will I gain or lose anything by raising the antenna above the ground
plane?

I'm hoping raising it would give a more circular pattern by getting it
above the roof line and above the 150/450 MHz scanner antenna mounted
on the other side of the trunk.