As a big fan of 6.5-284 and a big fan of coopers ( I have 5 ), I thought I would log in an give my once a year opinion. Firstly, I find all their barrels to be accurate but lower velocities and pressure signs before I near max loads. All can shoot 5 shot 100 yard groups under 0.75" with a load it likes.
I was fortunate with my 6.5-284 Cooper Phoenix to quickly find a load with 142 gr SMK and R22 that would repeat groups 0.3 to 0.4" when I kept the bullets seated 15 thousandths off the lands. I did not notice but I hope you are using Lapua brass. I have over 1500 rounds through the barrel and no degradation in accuracy....yet...but I know it is coming.
Where I will switch my direction is your comment on using Berger. I shoot F Class and just put my 5th barrel on my 308. I have found Berger bullets extremely accurate once you test the "bullet jump" for your barrel. I have had barrels that like 5 thou and some that like 30 thou. All cut with the same reamer on Kriegers. My opinion is to experiment with jump once you have measured OAL to lands with a great tool like Sinclair. My tool is 20 years old but I am sure you can still get one.
I hope you get yours to shoot well. Copper makes an outstanding product and stands behind them.
Your shooting friend from Canada.