I've experimented considerably with two .257 Weatherbys, one Vanguard Sporter and a NULA Model 28, over the past 15 years or so. I haven't found any one powder to be the magic solution, and have tried a bunch, including RL-22, RL-25, RL-33, Magpro, Magnum, IMR4350 (the powder Roy Weatherby used when designing the round), IMR7828, Retumbo and some others I've no doubt forgotten.
But the most accurate load in both rifles used monolithic 100-grain bullets (whether the TSX, TTSX or E-Tip) and Ramshot Magnum. In the Vanguard this would regularly group three shots in 1.5" at 300 yards. That said, I recently tried H1000 and got very close to the same results.
