I had much the same results. I did not try 115 Bergers but did try the 115 DTAC. The DTAC would shoot ok but nothing to write home about. The 105 Berger Hybrids really woke the accuracy up. I have 8 lbs. of old Winchester WMR which works very good along with Retumbo & RL33. I have not decided on a final load as both WMR & RL33 are neck n' neck. The velocities are close with the WMR clocking 3389 fps with the occasional ejector mark. The RL33 runs 3407 fps with no pressure signs at all. More testing is required for me to pick a final load.
Try several of the heavier bullets to see which one works the best. If you feel the need to buy powder I think that RL33 will turn out to be "THE" powder for the 6x284.