The compression driver has an 8° flare exit angle which matches the entrance angle of the horn. The horn gradually expands that flare rate, setting the wave shape and coverage pattern. When you install an adapter, this is changed. The 8° flare angle at the compression driver exit interfaces with a 0° section in the adapter, basically a short straight pipe, for some distance. It then goes back to 8° at the horn throat entrance and expansion continues from that point. This poor interface results in a discontinuity that causes internal reflections and it occurs at a very sensitive place in the horn.

In some cases, I would expect severe spikes in the top end, in others, it may not be as noticeable. You could always bolt it on, measure it and see. Might knock down some of the breakup modes, but then again, it might make them worse. Hard to say without measurements. It will almost definitely have an impact, and I would not expect it to be a positive one.