The US being aware of Japanese plans of attack has lots more impact than what happens at Pearl Harbor. It allows the US to be better prepared everywhere. Wake Island could be held, and the Philippines could be better defended. Since the US would pass on such knowledge to the British and Dutch, it means the entire Allied war effort in the Pacific would be better coordinated.
It really depends on how much advance notice the US receives. It's the difference between merely putting MacArthur, Kimmel, and Short on alert 24 hours beforehand versus a coordinarted Allied plan starting a week earlier versus major operational changes and reinforcements a month earlier.
Since the great success of Japan's initial offensive was based entirely on strategic surprise against unprepared opponents, eliminating both the surprise and the Allied unpreparedness greatly changes how the Pacific War unfolds for the first six months.