Winds that strong I find make deer more cautious when moving around. They will often stop and test the winds before they proceed. I think this is because the sound of the winds might hide noises that would otherwise alert them to danger.

As to the effect on a muzzleloader will depend on the distance you have to shoot and under what conditions. Gusts can effect you more then anything else. A steady wind might move you off the POA a little, but those speeds do not really worry me too much. Unless I was shooting across a 200 yard field or something like that.