Mark, we have been shooting the 165 matrix in 10 twist barrels out of the 270 WSM at 5000 ft up and have had excellent accuracy and no stability issues. I am going to have Brux cut me a 9 twist .270 for my next 270 WSM build(170 Berger should be sweet).
We are using Wyatt extended mag boxes in rem 700,s max coal is 2.995 and it is working great with the 165 matrix. We are using a b.c. of .650 at 2965fps and drops have been on to 1000 yds.