@rbarbosa: I have the impression that you are understanding the way the Decal System works very well. Without knowing more about the project and without having tested the performance of the Decal System on mobiles, it is impossible for me to give you a clear answer.
Computing the decal in the editor would be the optimal case from a performance point of view, but that certainly doesn't work in all cases. So if you want to compute them at runtime, the performance heavily depends on the number of vertices/edges/triangles the ground has onto which you perform the projection.
It is out of question that the computation of a decal is expensive, but with a simple ground geometry it may even work at runtime. But I can't promise anything.
Very vague answer, I am aware of that. In the case that you are testing it, it would be awesome if you could share your experience!