I would imagine that MAGIC decrypts at the time didn’t add much to the ONI’s knowledge base. At this point MAGIC is mostly just PURPLE diplomatic messages which would not given much information on ship dispositions.
In the spring of 1943, the Japanese were running monthly supply convoys to Attu and Kiska, usually two or three freighters escorted by a handful of destroyers. On the morning of March 27, a task force composed of one heavy and one light cruiser and four destroyers attempted to ambush this force but were instead ambushed themselves when they discovered that the Japanese had sent two heavy and two light cruisers up to provide support for the convoy escorts.
In what became known as the Battle of the Komandorski Islands, the US and Japanese Navies slugged it out for almost four hours in the last daytime gunnery duel in naval history that did not involve aircraft. Toward the end of the battle, not realizing that he'd crippled the American heavy cruiser (Salt Lake City) and one of the destroyers (Bailey), Admiral Hosogaya fears the arrival of American reinforcements and orders a full retreat. For his actions, he is relieved of command and forced into retirement.
On a personal note, the main reason I know so much about this battle is the commander of the Bailey was my grandmother's cousin, Lt. Commander John Atkeson. He received the Navy Cross for his actions in this engagement.