Implant to reshape maxillary tuberosity area after 3rd molar extraction?

Stephanie,
The short answer to your question, will having one dental implant in the 16 area make a difference in your outer appearance, is no.  If you are noticing changes in the appearance of your face after an extraction of a wisdom tooth, it is most likely weight loss, not bone loss or muscle atrophy.  There may be some nerve damage that occured during the extraction  that could cause your cheek to sag, but damage to the facial nerve would generally cause sagging of the whole side of your face and even under your eye.  
Getting the tooth out was  a good idea, based on your description.  It is incredibly rare to replace a wisdom tooth with a dental implant.  There are easier and less invasive ways to change your outer appearance. You may want to consider fillers (see plastic surgeon).  The easiest way would be to gain some weight, but that might not be the path you want to follow.
The bigger problem here is the tightness in your muscles.  My suggestion would be to see another oral surgeon, ENT, or dentist that has experience in treating TMD and other facial issues.  You certainly should have therapy to relax and possibly deprogram the muscles.  
Warm Regards,