Well, I'm going to take a half-step back. I do remember some tests by user Rntlee that did show an extreme underbore was causing a decrease in shot grouping/accuracy. However, this was an underbore in excess of .008 cal. Extreme underbore was causing both shot failure and shot inconsistency. It may be that user TimedFormula8 is perturbing those results slightly. If your shots are curving and you're not using an extreme underbore, upping the bore size of an insert is not going to fix that problem. However, if you're using a .676 or .678 insert on .688 cal paint, changing to a .680 or larger insert would improve accuracy slightly.
(all of this said, this response is based only on Rntlee's testing - I'm not aware of it having been duplicated by any other users for confirmation.)
I remember those tests as well. That correlates to Punkworks data showing that a safe underbore went up to about .006-.008 under.