One wonders how far they could have gone. Perhaps unify the area of the Saudi state and the gulf states today, but I doubt they could have taken Syria for long even from a weakened Ottoman empire: perhaps Iraq, which was harder to reach from Istanbul? Gunpowder empires: the days when a lightly armed cavalry army of the sort they led could make sweeping conquests in densely populated areas were over save under fairly exceptional circumstances. (And Muhammed Ali of Egypt would have kicked the Saudi's ass if they tried to take Egypt: his sons took the Saudis down on their own soil, after all.)
Bruce