Captn C,
The specs for the 7RUM from the Remington website say they use a 9 1/2" twist for the RUM which should suffice for jsut about any current 284 diameter bullet out there. $575 was a good deal.
One other suggestion: Don't get really hung up on BC. You said for now you are going to shoot from 400-800yds and you have a good platform to do that with this rifle and a good scope. But use the bullet that is most accurate in this rifle and gives you the terminal preformance you are looking for. I have seen guys run ballistic drop tables over and over before ever pulling a trigger on their rifle. And they make up their minds of what bullet they "want" to shoot rather than what bullet is the most accurate.
Then later on, they come this and other message boards asking questions about why they are not getting the accuracy they expected. Just keep an open mind is all I'm saying and don't get to hung up on the numbers. BC means drop... they build turrets in the scope for a reason. If one bullet drops a few more inches than the other, it only take a few more clicks to get your zero. But you are still zeroed. And the difference in velocity/energy at this range isn't worth worrying about.
Now once you get 400-800yds down pat and go custom and start shooting out past 800yds... then yes BC and velocity/energy come into play a little more. But until then keep it simple and concentrate on important things.
Good luck with your new toy!
Steve