But yes, it would have made a lot of difference to the war, as the Japanese now have an even greater logistical nightmare, especially when Chiang invades Indochina and Manchuria.
Well, Chiang won't be attacking Manchuria anytime soon, and I don't think he'd attack Indochina at all. He just won a huge victory with Japan leaving, and he desperaterly needs to rebuild his army and prepare for a showdown with the Communists and coopting the warlords.
This will presumably be a planned withdrawl, so Chiang will insist that Japan only leave occupied areas when nationalist troops loyal to him arrive. There is probably sporadic fighting in the areas around Yenan as Communists clash with Japanese troops, but Chiang won't consider that to be provocative, and the Japanese would be more than willing to fight. So most of occupied territory is given over to Chiang, who rules even more land directly after the war than he did beforehand.
He also has the return of the industrial areas in Wuhan, Nanking, and Shanghai, and all that customs revenue.
Given the limits of the new US-Japan war, he's limited in trading partners, so he'll need to reequip his troops with Japanese equipment (assuming the Japanese can spare any)! At least with control over the populated areas again, he'll have all that farmland and raw materials available to buy, so we won't see the ravaging of the Chinese economy that happened IOTL after the Burma Road was cut short.
I expect one to two years of consolidation before Chiang moves against the Communists. With improved economy, an orderly withdrawl, de facto Japanese support, no pressure by Washingtont to reform, and no Soviet aid to the CCP, I give Chiang much better than a 50/50 chance to defeat the Communists completely, and even if not, he'll still deal them a serious blow.
Victory should come just in time for Chiang to re-enter the war against Japan and invade Manchukuo as the US draws its Pacific Campaign to a close. Of course, he'll have to deal with a rather angry US and UK over his initial decision to accept peace with Japan, but I think it'll be something he can handle. In any case, Nationalist China will be much strengthened ITL than IOTL.
Of course, this is all fantasy as I agree with the other posters that by leaving China, you've ended the reason Japan and the US went to war.