I'm actually really unsure about ideal natures regarding many of the Gen 6 Pokemon... I think Impish is what they're saying is ideal on Chesnaught, but I could be wrong. But Synthesis is definitely something they say is a good move on Chesnaught, if you're unsure! So I think you're on the right track to competitive battling prowess for Chespin.
Also, you should! 6 and 5IV Pokemon are definitely nice to have. <3