It seems that my initial assumption that a 1/4 wave would lose any advantage because it would be mounted down low is correct
That's pretty much it. Look at this way, you won't gain much advantage either for the extra work involved to change antennas.
I believe you did your antenna mount correctly and will work as good as you would hope for.
The only exception would be if that is a mag mount because It's hard to tell in your pic. If so, you could improve the antenna bonding and other negative effects associated with mag -mounts and use a permanent trunk mount. I know drilling a whole doesn't sit well with many folks, but a mag mount should only be used as a temporary antenna.