I do not know if that velocity level is reasonable, or if that would be a velocity which would give maximum accuracy. A long barrel can be an easy way of maximizing velocity.
6.5x55 and 6.5x55 Improved are well thought of for F Class shooting. I am sure that 6mmm would work, although bore life would likely be shorter. .243 has a reputation of having a relatively short accuacy life for target shooting, and you are going to be using more powder. It has been suggested that the longer neck of the 6mm Rem. contributes to better barrel life.
I suppose it basically depends on your desire to use Lapua 6.5X55 brass as the parent case.