  Quantitative entanglement witnesses allow one to bound the entanglement
present in a system by acquiring a single expectation value. In this paper we
analyze a special class of such observables which are associated with
(generalized) Werner and Isotropic states. For them the optimal bounding
functions can be easily derived by exploiting known results on twirling
transformations. By focusing on an explicit local decomposition for these
observables we then show how simple classical post-processing of the measured
data can tighten the entanglement bounds. Quantum optics implementations based
on hyper-entanglement generation schemes are analyzed.
