Re: 16"-14.5"
General rule of thumb is about 50-75fps per inch. Lots of things can effect this, of course, not the least of which is chamber dimensions and even the barrel's internal finish. Some ammunition performs better than others in short barrels. I've always been surprised that the Mk. 262 ammo performs as well as it does from 14" barrels.
Having said that, I don't like barrels shorter than 16" on a .223. You do give up quite a bit and 16" gives up about as much as I can stand when compared to 20". I guess it depends on what you can live with. I don't much care for seeing .223 running at 2500fps. Individual guns can vary a lot, though, so if you don't chrono it maybe it won't bother you. ;D