762 ULTRAMAGA
WKR
The 6.8 western is not exactly the same as the 270 wsm.It's funny you suggest rebarreling a 270 WSM, because that is EXACTLY what the 6.8 Western is. It's an updated version of the 270 WSM. Winchester/Browning took the case, inserted a longer, heavier for caliber bullet, throated it for the heavies, and tightened the twist. What you suggest is exactly what they did at the factory.
The parent case is the wsm but it's shortened by .080 which buys you virtually nothing for working better in a short action, but it does reduce the capacity to that of a 7 saum.
In the end you have a .277 caliber cartridge that falls right between the 270 win and 270 wsm, and with the 6.5 prc this cartridge brings nothing new to the table.
The faster twist and saami oal for non handloaders, is really being over advertised as the "new and improved" when really its just the same old reinvented wheel.
I haven't played with the case but I suspect something like a 170 Berger would probably still need custom throating to maximize performance.
A custom throated 8 twist 270 wsm is still going to be a better setup, especially if its a rebarreled Tikka.
Heck a fast twist boring old 270 win throated for 170s should match it ballisticaly