Sorry, Too Fierce, but the vest does not sound like a good idea to me, either. You don't seem to want to hear that. I hate to dump on anybody who's willing to coach at any level, but don't you think that if this was a good idea, a ski coach or strength/conditioning coach with one of the world's better development programs or academies might be using it? And if it worked, perhaps they would all be on it?
These are men who have spent years, 24X7 working on developing the fastest and best in the sport, often with almost unlimited money and staff. Forget the academies in the US, think about the national sports academies in Austria, Switzerland, France, Norway, etc.
There's a reason why training at the highest levels is highly focused and designed to replicate race conditions. I am quite certain that if you ran this idea by any doc, PT, etc involved in the sport, they would tell you to stop at once. But aside from that, my advice {other than drills} is to try to train as you race. You're not going to race with the vest. I would encourage the earlier advice to separate the strength and fitness training from the on snow training. Again, a high level coach would never have you training with that vest.
If you need to get stronger, as you continue to grow, get on a good year round program to increase strength. Also do the same with quickness, agility, and overall fitness. In this sport you want a very strong core, and you don't need beach muscles. A nutritionist might have some hints, too. I know a number of guys who are about your size who have tried very hard to get a little bigger, and it's been tough. Much depends on your physiology. Those guys, though, ski at the NCAA level, at the NorAm level, and have done just fine at less that 150 lbs. So I wouldn't obsess over it. I think you can make big gains without being bigger. As long as you maximize your strength and fitness. And at your age, you will continue to full out a bit, I assume.
Your actual skiing, combined with your tactics, will make you faster over time as both get better. Better skiing, and a bit more strength will allow you to do things tactically that make a big difference. You'll be able to take a few more risks, etc. You be able to naturally generate more edge angle, and effect quicker transitions and switches.
As far as being a "power skier", that may be some of the issue as well. I was surprised at your comments in another thread about "skiing through" a pair of skis, which I assume means overpowering them. What you may find will yield better results is trying to develop the best touch and feel for the snow that you can. Generally speaking, my observation is that even at the WC level, the fastest guys know when and where to pressure the ski and just how much. Then they know how and where to release it. They spend as much time as possible in the fall line, taking it down the hill. When you take that the J1-2 ranks, you'll often see guys who feel that this power is fast, and they are literally digging in, skiing round, and losing time in every turn. I'm not suggesting straight and late, either. I'm suggesting that the right touch and pressure is key. Just a thought. I spend a lot of time around Eastern FIS races. The fastest guys have great touch, as well as strength.
When Bode was in his prime as a GS skier {where he still has most of his WC wins}, the reason that he was so blisteringly fast was because he had what most coaches felt was an uncanny ability to put himself in the fall line, quickly, and stay there longer. I know a couple of guys who have coached him, and the general comment is that his body could have been all over the place at times, but knees down he was generally perfect and doing things that nobody else could. If you're skiing with "power" as you describe, it's hard to get that touch.
It's also easy to get overamped, and get more tight than you realize. I've seen a lot of this, at your age. I'm not being critical, as you obviously care about this and want to do well and have fun. Just saying that if you feel that you're small and need to go out and just kill it with power and aggression to get even, it may not be the fastest strategy. Sometimes when you concentrate on just trying to ski really clean arcs, with enough pressure, and a light touch, you'll find that it's real fast.
Good luck and have fun. And please re-think the vest. I'd hate to see you do some serious damage. By the way, all things being exactly equal, the bigger guy is faster. In my experience with the sport all things are never exactly equal. The best young downhiller, where size matters, in the US is Andrew Weibrecht who is 5'6". Scoring on the World Cup. Marcel Hirscher is maybe a better example. 155 lbs. Winning on the WC. Work with whatever physiology you have.
Sorry to be so long winded. Hope you think about it.