I have found over time that best accuracy with the 260 rem has been with Rem 9.5 primers. I've got literally hundreds of targets shot with 3 different primers 210M, BR2 and 9.5's the vast majority are moa or better. The most targets that have accuracy in the .5 moa or less range are with 9.5,s.
I have never tried 215's or magnum primers in the 260 rem but I have read to many stories about the use of magnum primers with ball powders in 7mm08 to discount the possibility that excellent could not result. I would not load up a bunch until proven twice or more.
It depends on how stiff your powder charge is but I would drop back at least 1 grain and please post your results.
Good luck and shoot straight