The v-stacks from aftermarket companies (graves, Factory pro) are designed to replace the stock stacks. They are air plenums mounted in the air box and help control air velocity and feed into the throttle bodies or carbureators.
Where they help you, or my bike 03, is up top from 12,000-15,500. It flattens out the peak horsepower and holds it for more rpms. (top horse power is gonna have wider RPM range, not just hit top horse and then fall off)
Unless you spend alot of time in those rpm's they are basically pointless to worry about, but if you do spend your riding time in those rpm's, they are WELL worth it, and a cheap mod for what they can do. They will help a little without doing any remapping etc... but it helps the best to have good flowing exhaust and a little mapping work.