I am looking into getting my 300wm cut to 15" and have a break installed...
Heres what I have found from talking on this and other site....
Generaly speaking you can still get MOA accuracy from a cut 15" brl, you will need to drop the bullet weight to mid or lower weight
and your going to have to rethink your powderd as well.
The other thing you need to ask your gunsmith is his headstock dimension for cutting an Encore brl with the lug still attached. Most are going to be 19 3/4" ish.
You want him to cut & crown it in the headstock, they can cut it with a "chop saw" and drill press the crown, but thats probably not what you want done.