Underwood 10mm is some of the hottest 10mm ammo you will ever get your hands on. Right up at the limit of SAMMI specs. You can feel a massive difference between it and Double-Tap. It is some powerful stuff. I know because I have tried everything they have in normal JHP from 135-200 grains, and most of the stuff Double-Tap has in that range as well, on the same day too.
As to their other calibers, I can not speak to them directly.
And as to the comment about once-fired brass. Underwood is pretty new and pretty small so they may do some reloading. However, if you are getting reloaded brass they will tell you in the listing. Also I know for certain that they used to offer a service where you sent them your old brass and they reloaded it for you. I am not sure if they still do that, or if they reload in any way at all. I know all 800 rounds I have bought from them in 10mm were all new brass.