According to some popular stability programs out there the 10x with 300 SMK's shows a stability factor of 1.76 Shawn Carlock swears by the 10x for the 338 EDGE. Ballistically they are one in the same.
1.1 is concidered stable. 1.4 to 1.5 is prefered to allow for atmospheric swings. 1.76 is more than enough. That said, if you ever plan on shooting some of the newer mono metal bullets it may be wise to go with the 9.3 or so.
The routine accuracy testing for the 300 SMKs is done with a .338 Lapua, using 1x10" twist barrels; never a problem. I'd have to say the new Bergers should also do just fine out of the same twist. On the flip side, a little extra spin is rarely a bad thing.