Re: 7mm Twist
Your twist rate selection has a lot to do with what other rifles you own. My 7-08AI wears a 22" #2 Shilen Stainless Match barrel with a 1:11 twist. When I called Shilen about the project and told them I would only be shooting bullets 140gr and lighter, they recommended the 11 twist for higher velocity and better accuracy potential. I use my 300WinMg for any game that a 140gr 7mm can't handle.
The 10 twist will stabilize bullets up to 160gr. 7mm Weatherby Mags used to have a 10 twist, but they reportedly would not stabilize all 175gr bullets well.
Since I can easily get over 3000fps from my little 22" barrel and accuracy averages a hair under .5MOA, I couldn't be happier with my selection. Your desire for a 280AI with a 10 twist sounds good to me.