Score does affect whether countries increasing funding but the worst/best case is about 15% increase, you don't get any bonus for going over the threshold for rankup, most countries will just do nothing, and it's pretty random anyways, so as Solarius said almost unrelated to income.
Corpses could have a much larger store space or score could convert to actual money using performanceBonusFactor. But, really it's not a big deal.
Side note, I don't know why there's a performanceBonusFactor for flat money, and not one for income change. Score converting directly to money is not super useful when you already have Money Briefcases, and not very interesting compared to balancing the long-term effects of your "warcrimes".