I don't think that the ISIS pose any threat to the Saudis, as there is hardly any differences between the two sides. Do you remember the Grand Mosque seizure in 1979? Although the Saudis executed Juhayman al-Otaybi and his followers, the King gave more powers to the religious puritans in the immediate aftermath. The Saudi reaction to the ISIS will be the same. King Salman will make Saudi Arabia even more conservative, to make sure that no differences exist between the ISIS and the KSA.