Jesus, what a complicated subject. I did a load of work on this, and basically.... From a power point of view, it's not too much of an issue at all. On N/A, it can shift the curve up and down quite a bit. On forced, it has a much smaller impact. However, on one of the supercharged setups (possibly roots, i'd need to check) a larger plenum makes a big difference.

On a turbo car, the larger plenum helps to balance out the cylinder distribution, esp if u have an inlet biased to one or more cylinders. That's why you use a plenum spacer on a YB.

Rick.