I wouldn't worry about the loss of velocity, but I would be concerned about the muzzle blast. When I went shopping for my son's first centerfire rifle, we looked at both the Ruger M77 youth rifles and the Rem 700's. The rem 700 won out simply because I wouldn't put him behind a .243 with a barrel shorter than 20". Even with the 20" barrel the muzzle blast will get your attention.
Whatever route you go, load the 75 gr V-max over a max dose of Varget per Hodgdon's data, I expect you'll be quite happy with the results.
