Cringe or not, you're not going to notice too much difference, but you might be able to measure it.
By far, the largest factor in efficiency in mobile antennas are the ground losses encountered. Minimizing them is a paramount activity. Second is the coil Q. Once you get over about 300, there isn't much advantage in even doubling it (if you could, which you can't regardless of what you read in the advertisements). The third most important is over all length. A 9 foot antenna will be over 4 times more efficient than a 6 foot one.
If you jump through all of the hoops, and calculate the advantages you have as well as those you don't have, then you'd know why I said what I did in the first paragraph.
Think about this. The longer the antenna, and the higher the Q, it is necessary to put more effort into minimizing ground losses. Put another way, if the antenna is lossy to begin with, you won't notice much improvement even if you could reduce the ground losses to zero! Which you can't!
Just to put some figures on it, the ATAS, even hard mounted in the center of the roof, will be about 15 to 20 dB down from a decent screwdriver mounted on a fender or quarter panel. Mounted on a K400 or mag mount, and you can add another 4 to 6 dB to the difference. Trust me, 20 to 25 dB is a significant difference, especially when the bands are in the condition they are at the moment.