We numerically investigate atomic interferometry based on spin-exchange collisions in
F=1 spinor Bose-Einstein condensates in the regime of long evolution times $t\ensuremath{\gg}h/c$, where
c is the spin-dependent interaction energy. We show that the sensitivity of spin-mixing interferometry can be enhanced by using classically seeded initial states with a small population prepared in the ${m}_{F}=\ifmmode\pm\else\textpm\fi{}1$ states.