At the start of the war in July 1937, Chiang had eight divisions trained and equipped plus another 12 trained but not equipped. Unfortunately for China, the eight elite divisions were mauled early on during the fighting as Chiang engaged them in Shanghai. He thought that a defiant battle early on would encourage the Europeans and Americans to put pressure on Japan to cease fighting, but they were reluctant to pressure Japan either because the Depression or European politics preoccupied them. They took too heavily casualties early on and never recovered their original strength, although Chiang's central army overall (the forces under his direct control, not allied warlords) were still the best Chinese troops (other than the Guangxi Clique units). In retrospect, knowing the Allies would not intervene and the war would last many years, Chiang should have conserved his forces so they'd be available for the crucial battles inland.
So they had a good effect, but it was diminished because of the casualties they took. They were among the few units Chiang could really rely upon to follow his orders, and they acted as a mobile reserve to shore up his front. When Chiang effectively lost those units when Burma collapsed in 1942, their loss was a blow. Their presence probably enabled Chiang to continue to fight, but there weren't enough of them in 1937 so that he could defeat Japan. Hold out in stalemate, yes, but not win.
If Chiang had his entire central army so trained and equipped (I know the Wikipedia mentions an 80 division plan, but elsewhere I've read the actual number was 60 - about 20 trained and equipped per year for three years from 1936-1939), the war would have been very different. Japan likely would not have been able to land and advance from the Yangtze Delta, and its offensive in North China would have stalled at the Yellow River and perhaps even pushed back.
A huge advantage to Chiang is that with his central army trained equipped plus the divisions controlled by his most loyal allies, he'd be vastly superior to any of the warlords and he'd be able to manage the entire Chinese armed forces and domestic governance much better. It would increase the tax base, improve governance, and centralize the entire army's payroll and logistics. The warlords would simply not be able to resist Chiang. Chiang's goal was to establish a strong enough force to eliminate the warlords, and then after incorporating their lands under the Nanking government prepare for a war against Japan to reclaim Manchuria. So the idea was build the army from '36 to '39. Quash the warlords in '40 and '41. And then prepare for an offensive war against Japan sometime after 1942 probably after some kind of diplomatic maneuvering.
At the same time as the army plan, China was also expanding its industry so that it could produce a lot of the base materials and military equipment China needed. So any war that broke out in 1939 or 1940 would be unrecognizable compared to the actual war that began in 1937.