I would not go over 28". That is not a very big case. A 28" barrel should be more than enough length to burn that small amount of powder. When I was shooting my 6.5 x 284 imp. I had four barrels and never went over 28" and it has more capacity then the "6.5 x 47 Lapua" (I was burning 54 gr. of H-4831) You eventually reach a point of diminishing return on velocity.
Rich De