For a given magnet size, the higher the frequency the better magnet mounts of the same size functions.
At CB frequencies, a 5 inch magnet is barely able to do the job. At 2 meter frequencies, even a three inch magnet is far more effective than a 5 inch magnet at CB frequencies.
Also, just because it is grounded through capacitance does not automagically mean bad. If you get enough capacitance it is just as good as a direct connection. A lot of old broadcast antennas used capacitive ground systems to ground them to the earth. I wouldn't be surprised if some still do. They are essentially the same thing as a magnet mount today, although they are much bigger in size...
And finally, your version of "real ground" is apparently a direct connection? There are various uses of this word in the hobby/science of antennas. In modeling, for example, real ground is a specific type of earth that has certain characteristics. Also, many people see "real ground" as meaning, in your case, the earth under the vehicle as in many cases the vehicle chassis really only acts like a capacitive ground system with the earth below...
A friend of mine ran an Arrow so called Open Stub J-Pole on his mobile for a while. It worked fine. If you are insistent on removing such capacitive ground systems from your setup, it is something you might try...
The DB