  We study five-dimensional Yang-Mills theories compactified on an S^1/Z_2
orbifold. The fundamental Lagrangian naturally includes brane kinetic terms at
the orbifold fixed points which are induced by quantum corrections of the bulk
fields. The theories are quantized in the higher-dimensional R_xi gauges before
compactification. Using Ward and Slavnov-Taylor identities, an all-order proof
of a generalized equivalence theorem is presented. The theorem relates
scattering amplitudes of longitudinal Kaluza-Klein gauge bosons to amplitudes
of the corresponding scalar modes. Non-trivial sum rules among the fundamental
couplings of the 4D effective theory lie at the heart of high energy unitarity
cancellations. Using a novel coupled channel analysis, we derive an upper bound
on the number of Kaluza-Klein modes from perturbative unitarity. The bound
shows a very weak dependence on the size of the brane kinetic terms.
