  One of the much-debated novel features of theories with extra dimensions is
the presence of power-like loop corrections to gauge coupling unification,
which have the potential of allowing a significant reduction of the unification
scale. A recognized problem of such scenarios is the UV sensitivity of the
above power corrections. We consider situations where the grand unified group
is broken by the vacuum expectation value of a bulk field and find that,
because of the softness of this extra-dimensional symmetry breaking mechanism,
power-like threshold corrections are calculable and generic in many of the most
relevant settings. While the precision is limited by the presence of
higher-dimension bulk operators, the most dangerous of these operators are
naturally forbidden by symmetries of the bulk theory. Particularly interesting
and constrained scenarios arise in the context of higher-dimensional
supersymmetry. Our phenomenological exploration of SU(5) models in 5d, linked
in particular with more recently discussed orbifold GUT models, shows promising
results.
