According to Wikipedia, the Zircon is 10m long and carries a 300-400 kg warhead. This is not a small missile. I assumed it would carry a conventional radar homing head, but the Wiki article suggests that at Mach 9 there would be a plasma cloud around the front of the missile. That would greatly degrade microwave transmission and reception, good for stealth (defensive radars would have difficulty detecting the missile) but poor for the missile tracking a ship. However, I guess that there must be some integral terminal guidance as third party targeting at that speed would be flaky to say the least. But what is it?
As for CIWS defence, given the engagement range of CIWS and the speed of the missile, you might as well throw a handful of gravel at it. You have about 1 second from initial engagement to impact. Even if a remarkably lucky hit detonated the warhead, you still have several tons of debris travelling extremely fast, including some very large bits like the scramjet engine, coming inboard. Ouch!! Remember, there is still some doubt whether the Exocet that took out Sheffield actually detonated correctly, and the loss might have been due to impact damage and fire caused by the rocket motor.
The reason the Russians have always gone for very big, very fast missiles (who remembers AS-4?) is that they are extremely difficult to defeat. You have to engage them at long range to have any hope of protection and you own missiles have to be extremely agile to manoeuvre against a target with a closing speed of Mach 10 or more. Hence T45 and Aster. Let's hope we never have to find out whose technology is better.
Greg