The shear-induced isotropic-nematic transition is a generic property of liquid-crystalline polymer melts which is identified by the emergence in the isotropic phase of a strong birefringence above a critical shear rate. Although spectacular, this transition cannot be explained on the basis of a conventional approach (coupling with pretransitional fluctuations or with viscoelastic relaxation times). We investigate the asymptotic rheo-optical behavior of the shear-induced phase. The sample is an unentangled cyanobiphenyl side-chain polyacrylate. We show that the birefringence increases almost linearly and then saturates above a given shear rate depending on the gap thickness. Similarly, the shear stress versus shear rate curve exhibits for the same thickness and using the same substrate (quartz), an overshoot occurring at about the same shear rates followed by the stress plateau (indicating a stress-optical equivalence). In contrast, when the substrate is different, the stress-optical equivalence is no more valid. We interpret the overshoot and the plateau observed in birefringence and in shear stress curves as the entrance in a so far unidentified macroscopic sliding regime. The slipping mechanism is corroborated by the recent identification of macroscopic long-range correlations in unentangled polymer melt and contrasts with a description in terms of a nucleation-growth process as in wormlike micellar solutions.