For cases of overjet, how does Invisalign compare against conventional braces?

Your teeth DO NOT know what type of orthodontic appliance is being used on them. All the tooth will be aware of is force: magnitude and direction. Remember that Invisalign is just a different type of orthodontic appliance. ALL orthodontic appliances have to exert forces on to the teeth to cause tooth movement. Invisalign CANNOT overcome the limits of biology, such as how much bone is available in the jaws for safe expansion of the arches.

The decision to extract, or not to extract, for any case depends on several factors, such as the facial and lip profile (e. g. are the lips protruding?) the amount of overlap or crowding of the teeth, how much the teeth are flared forward with respect to the bone, etc. Patients with a substantial overjet would more often have lips that unable to close due to the teeth which are sticking out. In situations like this, extractions will result in a more aesthetic or pleasing outcome.