I've used both the GTR and the GTR Spec V, both fully modded. The Spec V did give be a little more hp after modifying, but nothing much to talk about for the extra cost of the base car.
However, I felt that the regular GTR handled better at high speeds than the Spec V (both fully modded, both untuned, i.e . all specs set to default). The Spec V had a tendency to understeer more, while the regular GTR seemed more willing to rotate in corners.
If I could do it over, I'd just get the regular GTR and mod it. Th extra cost of the Spec V isn't worth it to me.