OK, I can't watch the video, I'm on dial-up, but I have an idea of what's in it.
Chamber pressure should be lower than the original shell. Projectile weight is lower, or at least the same, and side pressure is reduced.
Accuracy should be worse than terrible. The Foster slug has reasonable accuracy due to the weight-forward design. That keeps the slug from tumbling. Your wax slug will tumble (some should make kind of a nice whistling sound).
If you're playing around just for the sake of playing, that is GOOD. I love to play with strange loads (keeping safety in mind). Just don't expect stellar performance.
DC