The bolded part is hardly up for debate, but it's also not the comparison we're making. I highly doubt that the ROC will be much more of a disaster than previous Chinese governments had been, as those too were effectively the same thing as you describe. Almost by definition, that means it's less of disaster than the PRC was. It's economic growth, even under a corrupt form of benign neglect, WILL be higher, earlier, than the PRC's. It will not semi-intentionally kill 50 million of its own citizens, nor will it stand in direct opposition to the United States, which means that it will probably get some aid. My belief is essentially this: it would be very difficult for them to do WORSE than the PRC, and not hard for them to be A LOT BETTER.
I can't agree with your supposition that ROC would mean less dead people than PRC. Yes tens of millions died in as the result of a failure of economic policy, but arguably as many people were saved through the successful immunization programs, the barefoot doctors campaign, and through the investment in rural education and food welfare programs. An ROC government which neglects the rural poor may not cause a massive famine, but it would mean millions of people dying from malnutrition related diseases, from infant mortality, and so on. It wouldn't be spectacular, and there wont be books written about it, but people will still die prematurely.
Furthermore, despite China having close ties to the US earlier, the economy wouldn't take off until the government has made the necessary investment in education and infrastructure, and favorable economic policies to take advantage of closer ties. India has had close ties to the West since Independence, but its economic reforms actually took place a decade after China.
From what I can see, there should be two policies where having the ROC surviving would make a real difference. The first is there wouldn't be the Cultural Revolution. Although this did not cause anywhere as many deaths as the Great Leap Forward, in many ways it did more damage as it targeted China's intellectuals. China's national spirit would be different, more traditionalist, the merchant class more "old money" than today.
The second major difference would be the One Child Policy. Back in the late '50s, the idea of population control first surfaced, but was rejected by Mao because he personally favored the idea of a more populous country. Had Chiang Kai-shek been in charge I suspect he would be in favor of population control since it limited the birth rate of the peasants, people who were nothing but trouble for him. An earlier population control policy would probably mean a more relaxed policy. But even so it would probably be extremely difficult to enforce due to the KMT's traditional lack of control over the rural population which made up the overwhelming majority.
This means one of two things. Either the population control program is ultimately abandoned, resulting in hundreds of millions more people, or the KMT would evolve to become a lot more authoritarian and efficient to carry this out. In that eventuality the KMT would evolve to look very similar to the PRC of today. If they fail and China becomes even more populated, the country would have a lower standard of living, more pollution and more malnutrition. The ultimate toll could be well in excess of the Mao years.
As for international relations, I expect the ROC to have good relations with both sides in the Cold War but inclined to be more friendly with the US, as it would be in a far better position to invest in the country than the Soviets.