Drop down to 6, 6.5, or 7 for starters....
Why? He already explained what he's doing and it requires a .257 and/or a .277 bore.
He want's to do something here rather than just follow the crowd.
External ballistics isn't following a crowd, nor is it an opinion...
But long range to some means working up a load distance to others....
6 or 6.5 would simply provide more options with fewer penalties is all.
But this is 'Merica....