To an extent, you are correct about the "2/3rds above the roof". sAe, I'm afraid that your statement isn't quite correct, as the coil, if properly designed and built (K40 and Wilson certainly are), radiates little to no signal. With a base-loaded antenna, as I'm assuming the K40 in question is, and as a Wilson 1000 mag-mount is, the "whip" above the coil does most or all of the radiating, with the majority of the signal radiating from the upper portion of the whip. For a center-loaded antenna, like a Wilson "trucker" series, there is some radiation from the short section below the coil, little to none from the coil, and the majority toward the top of the upper whip.
Antennas should be tuned when installed. Period. About the only exception is the full-size 1/4 wavelength (102") whip. There's generally no tuning provisions on those, and they are generally broad-banded enough to operate fine without tuning. A loaded antenna, be it mag-mount or hard-mount, needs to be tuned for best performance. If you're not looking form max performance, it doesn't really matter much. Just throwing it on there, with no tuning, should let you talk to your buddies on the trail a mile or so away with no problems. Despite popular opinion, running an untuned antenna will not damage the radio if you are running a stock radio. For mag- vs. hard-mount antennas, again, its a performance thing. The loading coil in a loaded antenna is used to make the antenna "appear to be" a 1/4 wavelength antenna from the radio's viewpoint. The 1/4 wave antenna uses the vehicle's body to make up another 1/4 wavelength, effectively creating a 1/2 wavelength "dipole" antenna. This is why its better to mount an antenna in the center of the roof. Mounting off-center "skews" the radiated pattern of the antenna, which should be "doughnut-shaped". With a loaded antenna, the body is still used to make up the other 1/4 wavelength the antenna needs to create a 1/2 wave dipole. With a mag-mount, you are "capacitively" coupling the antenna's ground to the body, which is not as effective as directly attaching the antenna's ground to the body. This is a pretty technical field if you get too deep into it; I'm trying not to get too deep and too technical. Also, just FYI, the popular 4' CB antenna, about the diameter of a finger, is whats known as a continuously loaded antenna, as the loading coil pretty much is the antenna, and runs from top to bottom. This is about the only situation where a properly designed and constructed loading coil radiates.
As for the Wilson .vs K40 issue, its a quality thing. Performance-wise, you probably won't see much, if any difference, aside from power handling capability. If you're running a stock radio, its a non-issue. The Wilson's are just built a little better, thats most of it. Don't get me wrong, the K40 is a good antenna, and is miles ahead of a lot of other antennas on the market. For the price you got it for, I'd run it, too! Of course, that being said, I'm running a 102" 1/4 wave whip, mounted on the left rear quarter, just above the "cut-line" that runs down the side of the body.
4-wheeling: because football, basketball, baseball, and golf only require one ball!
Confidence: The feeling you get just before you fully understand the situation!