I use 2 mag mount antennas on my Honda Fit. Both are Wilson brand antennas. I have the 2 meter mag mount and i havent checked the actual performance of the antenna other than the SWR meter in the radio (Icom 706 MiiG) it works perfect for me as i can remove it when i want or switch to my truck.
The other antenna ia a Wilson 500 for CB. I matched it to the Honda and i use it on 10 meters (28.400) I dont think most people get down to all the checking and stuff with the radio equiptment, just enough to get by. I look at it like if it is safe, does what you want and you are happy with it, then it must be the perfect setup.
For the power i put a 12 volt marine battery in the car and run the radio from that. This gives me a clean power source and i dont have to worry about killing the Honda's battery (which would take about 30 minutes)
Just my 2 cents.