But, I had in mind 230gr Berger OTMs. They work well out of my 1:10. And, I think Brian Litz (Applied Ballistics) sells premium loaded ammo.
Then by all means, use a 1:10 twist.
David Tubb shot a .308 with a 1:8 twist barrel shooting Sierra 250 HPMK's out at 2150 fps back in 1987 in the Palma Team Tryouts. Incredibly high in two ways, the arc in their trajectory and scores on paper. 'Twas almost like lobbing rounds from a mortar a thousand yards down range.
You do have a 300 win mag, so learning how to reload for that gun would be best I think. Once you get good at that, probably a couple hundred reloads or so, you will be able to understand more about what to build. You may consider 5-600 yards long range, which in my opinion is. Taking game over 800 yards does take great skills and lots of practice and buy a top notch scope for the 300 win mag. If you don't end up liking reloading or long distances you can sell both and get the bulk of your cash back. Buy an RCSB reloading package and a nightforce scope. You can't go wrong with that strategy and you are half way there. There is a good article on reloading and the best RCSB starter kit to buy in "Chuckhawks." I like what they write over there, look it up.
Another thought, there is a recent review about the new Browning x-bolt 308 varmint gun that is a tack driver in the review section here. Read it, the 308 is about the easiest to reload and there is good over the counter hunting and match ammo everywhere. It would also be a great out of the box long range practice gun. You can never have enough 308's. I consider the 308 win the best practice caliber re-loading or not for practice.
Dont get discouraged shooting 1000 yards is not as hard as it is made out to be. In sniper School every student is handed a rifle that is used by different shooters every class one bolt action M24 and one M110 SASS AR 10 platform. And Ammo that while it is Match grade With it being army Ammo It is defenitly not a hand load. Every student Qualifies out to 1000 meters with both guns And out to 1800 meters with a .50 cal. It is all about the right fundamentals and collecting data on you're gun. Seeing as how you are an experienced shooter you should have no problem getting to you're goal. But in my opinion I say 300 WM and set farther goals.