There will definitely be an effect of a longer bearing surface in terms of kinetic friction. However, whether you can actually measure a noticeable difference in terms of velocity is another question entirely. I believe a bullet with a longer bearing surface will run slightly higher pressure than a bullet of equal weight loaded to the exact same specs. Unless the difference in bearing surface length is large between two bullets of comparable weight, any slight effect on velocity/pressure you might observe can usually be remedied (if necessary) by slightly adjusting charge weight. In my experience, there are other considerations that might crop up with respect to bearing surface length that are more important, which include:
1) a longer bearing surface = greater friction and greater potential for jacket failure with long barrels, high velocity/pressure, especially noticeable in long and heavy .224" bullets such as Berger's 90 VLD, Hornady's 88 ELDM bullets
2) a longer bearing surface means a greater BTO dimension, which means more bullet shank down in the case/neck, and therefore a reduction in usable case volume. That generally means higher pressure at a given charge weight, possibly even compressed loads if the bullet BTO dimension is too long for the specific freebore of the rifle. This can be remedied by increasing freebore length to seat a longer (heavier) bullet farther out in the case neck while maintaining the same distance relationship to the lands.
3) an extremely short bearing surface may lead to balloting, or wobbling in the rifle bore, resulting in fliers and generally poor precision. In certain cases, this may be associated with bullet designs that have are extremely long and have a high BC for a given weight class of bullets. In pushing the envelope to generate the highest possible BC, it is certainly possible to shorten the bearing surface too much. I have encountered a couple bullet designs that I believe suffered from this issue.
If the rifle has sufficient freebore to load a bullet of a given weight class and bearing surface length (i.e. BTO dimension), slight differences between similar designs from different manufacturers shouldn't cause enormous changes in pressure or velocity. Because you should usually be able to adjust a load slightly to accommodate such small differences, the real question becomes which bullet design provides the best accuracy/precision? Only direct testing can answer that question.
In terms of generating greater velocity, slight differences in bearing surface length are not likely to make a huge difference. In my hands, pushing a load to the limit to generate slightly more velocity has not been beneficial. Unless it is possible to increase velocity by at least 75 to 100 fps (or more), the difference in wind deflection or maximum range is usually so small as to not be worth talking about. Further, loads that are pushed to the very upper limits in order to gain a slight bit more velocity sometimes behave erratically. They may shoot well one day, and poorly the next, if you can even get them to shoot at all. A far better approach is to select a good high BC bullet design or two, have the rifle throated appropriately, and develop loads that do not show excessive pressure. Let the BC of the bullet do the work for you.
