I had a more or less similar experience with a self-made spider, which seemed to perform better that the FA antenna next to it in the attic. However, the differences were not that different with regards to distance, but rather overall messages and a/c.
To the North, there is a brick wall, to the East clay tiles and sheetrock, to the West just clay tiles, to the South the wide expanse of the attic 
First, the atennae were side by side, then I put the FA atenna to a less favorable spot, away from the spider, the results were a little bit better, so I put it back and removed the spider, which lead a a further increase. So it maybe that the closenss of the spider or the closeness to another RPi with Wlan and USB cable influenced the reception of the FA antenna.
The FA atenna had the ProStick directly attached to it, after retiring the spider I used a pretty cheap 1m pigtail to connect it, if I remember correctly that lead to the biggest improvement.
I also changed from the Orange dongle to the Blue dongle. I live pretty rural, but there a still a few mobile Phone masts on a building close by, and some people nearby have strange antenna on the roof 
In my experience, the FA filter had little effect, probably also due to the difference in mobile phone frequency between the US and (Continental?) Europe. I have since added a cavity filter, which did not seem to have an immediate effect, positive or negative. Since then, over time the number of a/c and m per day have increased, but that is probably due to the air traffic picking up or maybe the re-arranging of mlat regions.
Experimenting with gain values has not brought me much success, at leas with the regards to the total number of a/c
Edit: Mine is not grounded either, it is attached to a wooden stand.
