I carried a custom 20" .375 Ultra for years. When shot over my Oehler 35P, it would drive 260s at 2900, 270s at 2800, 300s at 2650, 350s at 2500, and 380s at 2350. The boom it made was more pleasing than painful, although I tried to avoid shooting directly into the wind without hearing protection. As for accuracy, it shot like a varmint rifle.
If I had a similar rifle made up in .338 Winchester or Lapua, I think I would follow the same recipe. I doubt that a rifle with an 18.5" tube would be any handier than one with a 20" barrel, although neither would result in any disadvantage on the target whether paper or blood.