How many patients do you get who have a deficient enough chin that their face looks profoundly round from 3/4 view?

It is all about how deficient the chin is and the facial shape that is above it. (cheek and submalar fullness) The flatter or less full the cheeks are the rounder the face will look with a deficient chin in the three quarter view. Chin augmentation in a male, particularly that of a more square implant design, will improve the roundness along the jawline from the three quarter view.

 When the chin is recessive and there is a weak chin profile, a chin implant offers a significant complementary improvement to a facelift procedure to help  with facial balance and proportions. The soft tissues of the neck need bony structure for support, and a chin implant helps with this. For many examples of facelifts with and without chin implants, please see the link and the video below

The general answer to your question is yes, augmentation of the chin will elongate a relatively round appearing face.  Whether this will result in a more harmonious appearance of your features is to be determined by a careful examination of your face as well as evaluation of your dental occlusion or bite.

you have the right idea. Defining the jawline will alter  the overall facial shape and angularity. The jawline provides the balance to the middle face soft tissue; otherwise it does not drape evenly.