The present study explores a way to improve predictions of the mechanical anisotropy of textured polycrystalline aggregates. The underlying hypothesis is that grain-shape-dependent backstresses developed during the elastic-plastic transition influence the selection of active slip systems inside individual grains. Recently, a model was developed and applied successfully to electro-deposited pure iron with a columnar grain structure \cite{Delannay2011}. In the present study, we first suggest another definition of the boundary separation distance experienced by individual slip systems. Then, the model is adapted from the case of spheroidal grains, considered initially, to the more general situation of ellipsoidal grains. A combined effect of grain size, grain shape and texture on plastic anisotropy at yielding is illustrated in case of a rolled IF steel sheet.