I understand that you want to share the data you gathered, but I’d also like to suggest that “losing 1 fps” is essentially a totally meaningless measurement without additional context.
Going from 1000 fps to 999 fps is no real difference.
Going from 5 fps to 4 fps is a HUGE difference.
Also, “fps” depends a whole lot on the scene and your hardware, and additionally, it depends on where your current bottleneck lives – texture read? geometry transform? pixel shading? CPU simulation?
To quantify the cost of decals, it would be better to formulate the measurement as “on an otherwise empty scene that was fill rate limited, I lost 2% of frame time for every 200 decals.”
Regarding cost of decals: I estimate the cost of a decal to be very similar to the cost of rendering a simple static mesh of the same size in the same location. However, with the caveat that decals have to render after depth has been resolved, to support the “near Z testing” that removes the decal from areas outside the given decal volume. (At least deferred decals, which are the typical Unreal case!)