The amount of velocity you'll lose going to a shorter barrel is based on many factors and it really isn't always accurate to simply say you'll lose x-fps or y-fps per inch because often times, it can vary pretty wildly based on the quality/type of barrel used, the cartridge in question, environmental conditions and other factors as well.
The advantages of the longer barrel are more velocity, and thus, more effective range and better terminal performance at those ranges. By stepping from a 20" down to a 16-16.5" barreled .308, you'll likely lose somewhere between 125-150fps total based on my experiences across multiple platforms with multiple ammo types. Between 16-16.5" and down to 14.5", you'll likely experience a loss of another ~50-75fps. Again, for 600yds and in...I wouldn't sweat dropping from a 20" to a 16-16.5" barrel, but I'd personally avoid the 14.5" unless you are going to SBR it because of the need to permanently attaching your muzzle device to get it up to non-NFA barrel lengths and then, if you ever have to work on it, fully disassemble the rifle, change muzzle devices/suppressor types if you are using a suppressor mount muzzle device, it is costly and a real PITA!
