Two tablespoons of coconut palm sugar seemed just right to me while I was eating these mashed potatoes while still hot, but the next day the flavors had melded/intensified and the mashed potatoes seemed much sweeter. So if you're making this recipe to have leftovers or you don't regularly use much sweetener, you might want to use the lesser amount of coconut palm sugar.
Jefferson Adams recommended using oven-baked sweet potatoes for the best color and texture. I’m not doing much baking at the moment so I can't say if that will provide better results. I'll have to give that method a try when cooler weather arrives, but for now sweet potatoes that are boiled or baked in the microwave work just fine for me.