The appeal of the aftermarket barrel is better chamber support for warmer loads (extra margin of safety), plus it's easier on the brass if you decide to reload. I would agree that Underwoods ammo is the place to go for 10mm, but do keep in mind that their ammo is warm, and I'm not convinced is within spec. I've weighed charge weights (they use 800x for the most part) and it's definitely above "book" max charges. I'm not saying it isn't safe in the stock bbl, but if you're going to shoot warm loads with any regularity, an aftermarket barrel isn't a bad option. It's a mixed bag, some shoot warm stuff out of their 10mm, but when I had a 10mm, it didn't take no time to see brass bulging like crazy.
Congrats on the 10mm, I'm not convinced it's notably better than the .40 especially on game that's within reason for a pistol, but to each his own.
"Condemnation without investigation is the height of ignorance."