I tried a 45mm stem and found it way too short. You lose traction in corners with the shorter the stem. It felt good for jumps, but that's it.
I think 60-80 is ideal for the pack for someone looking to do more freeride than XC-AM.
I wouldn't go any shorter than 80-100 for XC and AM.
Just going 20mm longer on a stem can have a dramatic effect on increasing traction in corners.
Another thing, wide handle bars are underated. Most people think skinny handle bars are great because they don't clip trees and can steer quicker. On the other hand though, skinny bars make your bike incredibly unstable in corners and rough terrain at high speeds. They are especially bad for drops. Wide handle bars will help keep your bike going where you want it, thus much better at holding a line in corners and drops. Wide bars also give you much more control over the chamber of your bike. Landing sideways with skinny bars is going to be hard because you have no leverage over the yaw and chamber of the bike when you impact the ground. Wide bars will help force the bike to realign to your body's trajectory, not the other way around.
I run 700mm wide DH bars, never once hit a tree with them. Cornering is vastly improved over the 630mm bars.