Well...... I did do the match today, and I did wear it. It was better than wear nothing, and it did help. The two things it did provide was padding for the sling, and no shoulder slippage. The jacket is 16 years old, so it is broken in, so the leather is definitely not slick anymore. If it could have given me support by being tighter thru the torso, than it wouldn't have been half bad. My scores were pretty much the same as the borrowed, ill-fitting, basic shooting coat. It also provided no torso support as well.
So in my opinion it helped, better than nothing, but better choices out there. When I can afford it, I will definitely be getting a proper coat. The good thing is I am new to the sport, still learning, and having fun.
Forget to mention that it was 85 degrees today, and crazy humid. It was hot wearing it, but like I said, no slippage issues.