Just to back up Q.G. on this: a guy in a german forum ran a simulation of a 35mm format fully corrected 50/1.8 with and without extension rings. The results with the extension ring were downright depressing (field curvature, chromatic aberrations). Lenses are mostly corrected for infinity focus, where incident light rays are nearly parallel. As you focus closer, these light rays are no longer parallel and the correction is way off.
Even Mamiya seems to be aware of this, since they market a 140mm macro lens with is supposedly optimized for close range.