You are right about the general rule of thumb being plenum volume should roughly equal engine displacement, on a normally aspirated racing motor. This prevents the engine from bogging during closed to wide open throttle transitions, like on the start line in drag racing. It also allows better top end power when maximum air volume is required.
Increasing the plenum volume can sometimes decrease air velocity, which in turn limits throttle response and messes with A/F ratios at lower RPMs. Point being, if you spend a significant amount of time at WOT, or need the motor to react quickly from idle to WOT, lots of plenum volume is good, but not so good if it comes at the expense of velocity.
Plenum volume is much less important if you are using any sort of boost.
Hope this helps.
Garry
