Yes, armor weight decreases WPF.
So it decreases damage received from wpf, and swing speed accordingly.
Formula is really complex / will not be posted. I believe gaffer posted somewhere the gist of how much it ends up reducing it somewhere, go look around.
There are no magic numbers or anything, although 7 armor weight and less should experience no penalties.
tl;dr don't worry about it. Slight damage and speed decrease from more armor weight.