I bet it does make a little difference as a guess. It a piece of metal that is not symmetrical in the reactive near field (albeit a part of the antenna itself)and that will almost certainly make a
small difference. However it is difficult to know for sure whether it will be adding something or taking away.
I have heard the same about matching stubs on GM style antennas but never noticed it myself.
We must recall that conditions are the
king maker when it comes to DX.
However I suspect the difference may be unnoticeable. It is a relatively small piece of metal compared to the entire structure of a 8.5m long antenna.
We do know position of antenna with relation to ground plane can and does make a difference for antennas requiring a ground plane... i.e. broadly current fed. (or capacitive reactance at feed point of a 5/8th wave)
A car under a mag mount that does make a difference. And in the past I have oriented a vehicle to face a specific direction when running either whips or 1/4 waves on my car. Forming a little gain in one direction.
That is crudely shown and approximated here. But a vehicle is a BIG chunk of metal and greatly improves performance of a whip.
![Sirio - Sirio Vector 4000 Post-2261-16181713360756](https://files.mygmrs.com/forums/monthly_04_2021/post-2261-16181713360756.jpg)
It's not beam gain but I think you may get 1-3dB in the direction where most ground plane metal is.
i.e. mornings I face east, evenings I would face west following the suns path a little.
I would not sweat it and put it up without worrying.
Have fun with that I would like to try a Vector one day. Back in the day when you saw those you knew it was a serious CB station ! They look the business as well. A real neighbour anxiety inducing antenna.