There are a few things that need to be considered in the Iraq conflict. Firstly, that Iraq as we know it is perhaps over. Islamic state (formerly known as ISIS) is a force to be reckoned with and has already obliterated the border between Syria and Iraq. And they are here for the long haul. Taking back Mosul will take at least six months and the momentum with which they are moving is staggering. The Kurds are waiting for IS and the Shia militias to eliminate each other. They have increased their defenses around the oil rich area of Kirkuk and are hell bent on independence. They are ready for the fallout and they hold most of Iraq's oil.
Iraq is falling apart and Saudi tensions over the issue is a clear indication of this. 30,000 troops have been stationed on the border by Saudi Arabia after Iraqi forces withdrew from their positions leaving Syrian and Saudi frontiers exposed. Iraqi forces withdrawing has been refuted by the Iraqi minister's military spokesman, Lieutenant General Qassim Atta saying that the "fake news" aimed at hurting national morale. Yet, the Saudis felt the need for standing forces. And even if the border is well defended, Iraq's border with Jordan may provide an easier route into Saudi Arabia. Jordan may well be next on the list.
Of course, the Kingdom is already cognizant of these facts. As much as it despises the Shias, it doesn't wish to see the government fall to an IS caliphate making their monarchy redundant. But for now, it does not seem that IS wants to aim for the Kingdom. In fact, its the Iraqis who are upset with Saudi Arabia for bankrolling Sunni militancy in the region leading to their country being torn apart; claims that Saudi Arabia dismisses trumpeting their massive $500 million dollar pledge of humanitarian aid for displaced Iraqis. Iraq has not been a viable country since its colonial inception. Partition now, seems to be the only logical outcome. 
Thinking of the ultimate aim of the IS caliphate, more than a face-off with Saudi Arabia, it will want to take the fight to Israel. Will it have more support then? It is likely that it will, but it remains to be seen where Saudi Arabia puts the rest of its chips. What no one wants is an Iraq shaped hole when Israel arms up. Though their policy is a blur right now we all know a little too well, what the US position will be once Israel is thrown into the mix.