This is just a rough guess but the 9mm was designed to run in a short pistol barrel so they use fast burning powders like Unique, Bullseye and W-231. The pressure curve peaks very quickly then drops off just as fast.
So I would think that if you are getting 1,200 FPS in a full sized pistol I would guesstimate that you would only see an additional 175 to 200 FPS gain in velocity in an 18" barrel since the pressure will drop off dramatically after the bullet travels 10" or 12" down the barrel.
See if you have a shooting range nearby that has a chronograph that you can use to verify. Or better yet, get one for yourself, they aren't that expensive. I would love to see the results!