Just got done loading 110g accubonds with 42g of imr4350. That particular powder comes pretty highly rated for 257 Roberts in my online research. I also loaded some 117 round noses with 41g and some 100g partitions with 43g to test at the range.
+1 for 110 Gr. Accubond sitting over ~ 41.8 gr. IMR 4350. My dad has shot the round nose out of the Bob for years and tells me that my "fancy" bullets won't kill any faster. He's probably right but I like my "fancy" bullets and the way they shoot.
As for factory loads, I was gifted a box of Hornady 117 gr. Superformance. I think I shot ten rounds and they grouped well. However, I'm not a fan of the SST and put the box away for a rainy day. My plan is to shoot them at the range and reload them with the previously mentioned 120 gr. Partitions as an elk load.