The answers to these questions are not as simple as they may seem. For example, a 2" longer barrel doesn't necessarily mean any increase at all in velocity. Sometimes it might be only a few fps faster. The barrel will tune in where it tunes in.
One thing that can be definitively stated is that bullet weight, barrel length, and achievable pressure (velocity) are closely linked. For example, in .308 Win there is a very nice accuracy node for 185 Juggernauts over Varget at ~2725 fps or so from a 30" barrel. Trying to hit that same accuracy node with a 28" barrel will require higher pressure than a 30" barrel, even though the final velocities from 28" versus 30" barrels aren't much different. Likewise, hitting the same node with 32" barrel would require even less pressure.
In terms of velocity, with commercial ammunition you will generally find the trend to be somewhere in the neighborhood of 15- 25 fps increase in velocity per inch of barrel between lengths of 16" to 26" or so. Once you start using a barrel longer than about 26" with commercial ammunition, the velocity gain per inch of barrel typically drops noticeably. In fact, with barrels in the 28" to 32" length you may not even observe much of an increase in velocity. The reason for this is likely to be the powder/primer choice. Commercial ammunition loads are not necessarily optimized for 28" to 32" barrels. The powders used may effectively be completely combusted long before the bullet exits the bore.
To some extent, reloaders can compensate for some of the undesirable factors observed with commercial ammunition and long barrels by careful selection of powders and primers, and charge weight testing. However, there is a limit to the number of good powder choices available within a given burn rate range. As such, there is no guarantee that a 32" will provide velocity that is significantly higher than a comparable 30" barrel with the same bullet, primer, powder, etc. In fact, any detectable increase in velocity may be less than the ES/SD values for the two loads. Alternatively, a 32" barrel in just the right circumstance might give you an extra 10 to 20 fps over a 30" barrel.
The more important question is whether such a modest increase in velocity from a 32" barrel will make a measurable difference in a shooter's score. The answer is that even very highly skilled shooters would find it difficult, if not impossible, to accurately quantify the effect of a 20 fps increase in velocity in terms of reduced wind deflection and higher scores. My point here is that the main reason some have chosen to go with a 32" barrel length is not about increased velocity and a possible performance gain, it's more about the fact that you can usually hit the same velocity at lower pressure with the longer pipe. That can represent a significant increase in brass life under the right circumstances. It may also allow loading a higher weight bullet that would cause excessive pressure issues when attempting to hit a certain accuracy node with a shorter barrel.
Of course, an obvious trade-off with longer barrels is the added weight. So many shooters will go with the minimum barrel length and chamber specs that allows them to load their bullet of choice without having to run the pressure up too high. Barrels of 30" to 32" length are probably more common, but 28" or 29" barrels are certainly not unheard of. Quickload reloading software is actually an excellent tool for making predictions such as the effect of barrel length, in silico, as an aid to the decision-making/buying process.