To get an idea of the effect of stacking, I mounted a Godox V1 flash on a D70s body with a 19-35 Tamron lens set to 19mm. I then photographed a blank ceiling from a distance of about five feet. This sequence was shot with one mounted grid (upper left), followed by a second grid (upper right), until I reached a total of four grids (lower right). No effort was made to adjust the exposure because I was only interested in seeing the shape of the highlight and the quality of the shadow edge. It is not as tightly focused provided by a fresnel lens, it is nonetheless a very smooth and symmetric highlight, a useful capability to have tucked into a lighting equipment bag.
I ran a quick series of selfies using a camera-mounted bounce flash as my key light and the gridded Godox flash to produce a hard round highlight on the background. I set the Godox to trigger optically, and placed it on a light stand about four feet from the wall. As before, the shots were fired with the least restrictive configuration (one grid) on the upper left to the most restrictive (four grids) in the lower right.
If you're wondering about the apparent size of the highlight, you need to remember that the flash to background distance is shorter in the selfie sequence than that of the ceiling shots. I imagine that when I start using these grids in the field, there will be a lot of adjustments to the location and the size of the highlight before I get the desired results. I anticipate a lot of both chimping and head-scratching if I am to get the subtle splashes of light I can see in my mind's camera.