me and a few locals did a quick unscientific test today, my i10k vs a new gainmaster on the same short mast, feedline, radio ect, we used my jfk foxhunting radio with 4-1/2" external s-meter and rf gain up full,
using 27.76125mhz fm no audio signals were stable ( no skip no flutter )
the antenna not under test was layed on the ground away and @ 90 degrees to the antenna on the mast, we swapped in a timely manner while everybody waited to see the results,
the tenths of s-units are our best guesstimation of what the big meter was reading on our end,
tx signals are low due to the receiving stations reducing their rf gain to make any change in signal easier to observe,
Way to go Bob, but I sure wished you were able to test your Vector too.
How high were the antennas and how far away were your contacts?
I just posted a recap on all of my 2010-2011 signal reports in another thread the other day, and I was surprised to see the GM so far down the list, albeit again all the antennas were very close together in RX signals. Even so I think, for several other important reasons that the GM is by far, the best CB vertical I have here.
I recall when I first got my GM, I set it up right away on a 10' foot mast and attached it to my fence for support, and it didn't work worth a hoot for me. In fact it was obvious to me that my AP and A99, which I had been playing with a little earlier, mounted in the same place, both responded better. I think I posted about this back then, claiming that the GM did not respond well low to the Earth. Seems like I remember someone else was saying the same thing about that time. I think Shockwave also told us that even though he wasn't initially impressed with his GM at first either, there was no problem with the GM being mounted low in height. However, I don't really remember if he told us why he was not impressed, so I don't know. In my case I thought the problem was due to the fact those antennas were not higher than the highest peak on my roof...which probably blocked off some of the signals. But after I moved it out back, I still didn't sense good performance, plus my bandwidth curve for it, mounted down that low, looked a mess as well...and
I thought that strange. IMO, most antennas mounted down that low seem to flatten out the band width curve due to losses, and make the curve nice and smooth, but in my case that is not-so with the Gain Master.
So, if you were below about 20' feet and had that big old I-10K was laying below it, like I noticed in my 2009 testing with my AstroPlanes and they were higher at over 20 feet high, I would think that might really ill-affect the performance of the GM. IMO, since the choke works so well otherwise maybe the GM is very sensitive in that area as a result.
I remember you telling us a few days ago how you layed the test antenna down in some particular position, and at the time I wondered why you felt that necessary to even mention, especially in light of the fact we discussed my experience, doing something similar, several times over the last couple of years. Were you testing to try and duplicate my experience to see if such was duplicable? Maybe you remember, that I was really surprised at that response I noticed.
Personally and in my experience with my I-10K and its potential to produce common mode currents, I'm not surprised that you saw better close in local signals with the feed line being an active part of the antenna system. I think that is also why I see good results with my A99 and why so many see good results with their Imax setups.
We now can quantify dollars to S units for vertical 11 meter antennas. 
