Brass price is a consideration here.
The 338/378 WBY brass is most expensive at $2.00 each.
The 338 Lapua Brass is next, at $1.30 each.
The 338 Ultra is even less, at $0.50 each.
The 338 Lapua brass is the strongest and will last much longer than either with proper annealing. It's consistancy is better, and especially if you don't neck turn this will be an even more important consideration.
Forming brass in the die before shooting it would only need to be done if you are lengthening the neck by pushing the shoulder back.
If you are blowing the case out to take out excess body taper and/or form a sharper shoulder, no problem, just seat the bullet hard into the lands to create the zero headspace condition and fire form them.
Depending on the Improved version of the Lapua, and there's a few designs too, your forming may be different, as well as with using other parent cases too.
The case will still center very well with the double radius shoulder Weatherby uses. If the die is set up properly, the forward edge of the belt never contacts the chamber, and it headspaces off the shoulder just as any non-belted case does.
At 14 lbs, I'd go for the brake too.
Try this for an online ballistics program.