The only issue at that range would be the speed. You need to check your ballistic calculator and make sure the velocity is still supersonic (about 1,300 fps). I would also be concerned with the FMJ as the back of the bullet has lead exposed and may cause issues with accuracy, but if the load groups 1 MOA at 100-200 yards, you may be okay. If not, try a match grade bullet as long as it is not a 168 gn SMK. They do not perform well past 600.
As far as the lower SD, 10 is not bad especially in a factory rifle. Anything in the single digits is good for MOA at distance. You can sort the brass of you like. If you are using Lapua brass, you really do not need to uniform the primer pockets or flash holes, unless you feel better about doing it.
You are going to get your biggest gains by changing primers (experiment with this. Wolf is good, and basically any match primer. I use CCI BR2)
Also using the same lot of Varget.
And neck tension is critical.
Good luck.