They're strong enough to beat accomplished Gym leaders. We're also bound to have a few more evos by the time the league begins anyway.For a believable league victory, the Pokemon each need to have justified strength. The Pokemon Ash has trained in Unova are not strong - Pikachu and the reserve Pokemon he barely trains are, and are the only reason he gets by. Having weak or inconsistent rivals and not having full battles up to this point helps hide that.
I agree, I think Ash's skills will surely shine in the Eastern Unova arc pumping him up for the league.I hope that either the World Tournament is before the league and provides for significant battle training to make a justifiable league win, helped by Dawn's contest battle style helping to show Ash's new Pokemon of those skills, or that he fails miserably in the league and goes there for redemption.