I killed a rock at a little over 900 yards with 2 shots from a 7mm STW as a demonstration while hunting caribou. (The rock was about the size of a small house but the facet I was shooting was about the size of a 4 pane window.)
I'd guess that the rifle you're discussing was setup for the 140 class bullets, but that's just a guess.
It was a factory Rem 700 in 7mm STW with a Leupold Vari-X III in 4.5 x 14 I believe. I was shooting 140 Nosler Balistic Tips that were placed on a factory case and powder after the factory 140 CoreLok was removed (case resized of course). I wanted to try the STW and thought a Caribou hunt would be a good place (caribou aren't very tough, they can be killed with a stale donut).
You might want to try a Cauterucio 156 VLD. They do very well in my 7mmRUM with a 30" 1:9" twist. I've settled on Cuat. 176s, but the 156s grouped as tightly (@ 3.25" at 500 yards). Both have extremely high BCs.
I agree with Dave, the 140 class would be the likely mid-range, but at the slightly lower velocitites in an STW compared to a RUM, that 156 just might be the ticket for you....