Since it has a 20" barrel, we could not go CAR length. When new, it would not function with the fast powders like H110 so I had to use the slower Re#7. The longer barrel got me the same velocities with Re#7 and 20" as I get with H110 and a 13" barrel. After the rifle had 500-600 rounds through it, it started functioning just fine with the H110 loads, but the longer barrel did not give me an increase in velocity over the 13" barrel. While the 20" is functioning with H110 now, IMO it is operating right on the edge.
That is why I never recommend a midlength gas with the .458S. On shorter barrels you lose velocity by having to use slower powders and with a longer barrel and slower powders, while you may regain the lost velocity, you do at the expense of a longer barrel and yet not gain any velocity if you can use faster powders once things smooth out.