If I am a 17-year-old, 165 lb pitcher, and can consistently throw 79-82 MPH for my fastball, if I get up to 200 lb with 12% or less body fat, should I be able to throw in mid 90s or at least the 90s?

Bodyweight has been directly correlated to fastball velocity.

Relationships between ball velocity and throwing mechanics in collegiate baseball pitchers

I touch upon this point here: “As a general rule of thumb, our Sacred Heart pitchers will gain 2-4 mph a season during their 4 years in college. As much as I would love to say  it’s just training, there are  numerous reasons why we get increased  ball velocity each year including mechanical improvement, growth of body due to puberty, increased  muscle mass, and improved body  awareness/muscular coordination.  The interesting trend is that most of  our athletes tend to add anywhere  from 5-15 lbs of bodyweight each year. Does 5-15lbs = 2-4mph on the mound?  Maybe.”

Why All Baseball Players Should Be Using Creatine

With my pitchers, we have the following hierarchy/system:

1) Mechanics Rule All: Increase mechanical efficiency your energy leaks will decrease and velocity will go up.
2) Increase General Strength: Building a foundational base strength is not only performance enhancing, but done properly will decrease the risk of injury.
3) Increase Muscle Mass: More muscle mass, the more potential to apply force.
4) Increase General Force Production (e.g. Power): Allows us to tap into more high threshold motor-units to produce more force and increase IIx muscle fiber.
5) Increase Skill Specific Power: Explosive training in a transverse plane. This could be med ball throws, weighted or  under-weighted ball throws, long toss, flat ground or even mound work.

We have had many mid 90MPH pitchers (injury free to boot) as well as some whom have touched 99+MPH. Some of these athletes have gained upwards of 50lbs of very clean weight (still viable abs) in less than a year.  Everyone is different based on genetics, work ethic, movement capabilities, diet etc.

If you are mechanically efficient, have relatively long levers and can increase body mass to 200lbs and force production increase proportionally it is very possible. This information as well as programming for this goal is covered in detail in Building the Perfect Pitcher.