From: Near Portland, OR
Yamamoto was sort of a bipolar strategist. Sometimes he was brilliant, other times he was terrible.
Throwing air power at the South Pacific was not as bad a decision as the whole Midway campaign. Up to that point, Japanese air power had proven to usually be superior to the Americans, so it was not unreasonable to believe that throwing more aircraft at the problem was not going to break the back of the Cactus Air Force and enable Japan to take back Guadalcanal. The US changed the calculus by learning from earlier mistakes and adjusting doctrine accordingly.
IMO, Midway was top to bottom a crazy idea.
Back to the original question, by early 1943 the die had been cast and Japan was certain to lose the war. From the start the only way the Allies would have lost is if they had lost the will to fight, but by 1943, that will was solidly set and it was obvious to anyone who understood American production abilities that the end was only a question of when.
Yamamoto may have pulled off a nasty surprise here or there to create to extra casualties, but ultimately, the war would most likely have gone down the path it did on pretty much the same timeline.
WitP AE - Test team lead, programmer