Savage makes an 8 twist for the 6br. I'm sure if you wanted an eight twist 243 they would oblige your request.
The .585BC DTAC falls below 1000 ft/lbs at 775 yds That's a moly bullet at 3150. Obtainable but near the edge.
At 900 yds your going below 1800 fps which is where reliable expansion becomes a factor. At 1025 yds your going below 700 ft/lbs which is another barometer for reliable expansion/transfer.
A 175gn 308 at 2650fps falls below the 1000ft/lbs at 650yds It falls below the 1800 fps mark at 500 yds. Lastly it's below the 700 ft/lbs at 850yds
The 300 wsm
shooting the 180mk at 2950 fps hits the same 3 parameters
700ftlbs 1025 yds
You can use a bigger bullet in the wsm but you start to intrude into the power space in a short action due to magazine length.
Using this data you could engage a deer sized target even a caribou but you had better be a half minute shooter at 1200 yds to do it. Not from a bench mind you from a field position.
With a bigger gun you will have more kinetic energy because of the larger frontal cross section of the bullet so you will have a little more lattatude in your hits. Meaning you need to be a 3/4 minute shooter at the same 1200yds under the same field conditions.
Shooting live weight animals under 150lbs I'd say yes on the 243. Mule deer, big whitetails of the north/midwest I'd want a 264 caliber or bigger. Driving an over .525 bc bullet at 2950 or better. Recoil would be my limiting factor as to what I ended up with.