  In order to quantitatively test the ability of averaged inhomogeneous
cosmologies to correctly describe observations of the large scale properties of
the Universe, we introduce a smoothed template metric corresponding to a
constant spatial curvature model at any time, but with an evolving curvature
parameter. This metric is used to compute quantities along an approximate
effective lightcone of the averaged model of the Universe. As opposed to the
standard Friedmann model, we parameterize this template metric by exact scaling
properties of an averaged inhomogeneous cosmology, and we also motivate this
form of the metric by results on a geometrical smoothing of inhomogeneous
cosmological hypersurfaces. We test our hypothesis for the template metric
against supernova data and the position of the CMB peaks, and infer the
goodness--of--fit and parameter uncertainties. We find that averaged
inhomogeneous models can reproduce the observations without requiring an
additional Dark Energy component (though a volume acceleration is still
needed), and that current data do not disfavour our main assumption on the
effective lightcone structure. We also show that the experimental uncertainties
on the angular diameter distance and the Hubble parameter from Baryon Acoustic
Oscillations measurements -- forseen in future surveys like the proposed EUCLID
satellite project -- are sufficiently small to distinguish between a FLRW
template geometry and the template geometry with consistently evolving
curvature. (Abridged)
