When I got my SEUC I did a lot of testing with trying to enhance the CB with some moderate improvements. I got better cables, played with lengths, with and without the coil, new vs old style HD antennas, etc, but the best improvement I got was changing to an aftermarket antenna. I bought about 6 different models and did all sorts of range/quality tests with a meter, and with a friend stationary at my house while I charted the distance on a map. I even talked to an electronics guy about upping the wattage, and there was only so much you can do with the crappy HD unit.
Sierra makes an adaptor that allows you to use an automotive antenna. I tried a bunch of different combinations and the best possible performance was a big ass Firestik, removing the AM/FM antenna, that will sap perfomance. It looked mighty out of place, but it worked good. However, I learned the hard way while 1/2 way through Canada a couple of years ago that the stock Harley mount just isn't up to the task of supporting these heavier antennas, and somewhere it snapped and I lost the whole shebang.
Anyway, after replacing the mount, I then backed into using the lighter Firefly, which also didn't look so out of place. However, it didn't really work all that much better than the older version of the HD antenna without the coil. After all of this monkeying around, I lost interest and just left it like that. When a few of us ride, as long as we're still in sight, we can probably reach out to a good mile or a little more. However, we usually stay much closer, so it hasn't been an issue.
The bottom-line is, without the gound plane of an automobile, you will just never get near that same kind of range. Imho, the effort to double the performance from 1 to 2 miles, especially in the middle of a desert is worthless. It is what it is.