I recently took the plunge and got myself the 9wt TFO BVK.
Online 8wt shootouts point out the higher swing weight of the BVK despite its low weight, so I compared the new (4pc) rod to my old (3pc) 8wt Redington.

My kitchen scale produced the following weights and swing weights respectively:
Red-> 135g, 300g.
BVK-> 119g, 370g.

Swing weight was measured by balancing the rod on a fulcrum directly under the middle of the grip while pressing down on the butt to keep the rod horisontal. What I am curious to learn about is the contribution of additional ferrule to the higher swing weight, or does the fact that the BVK has a faster action also result in more mass in the blank near the tip?

It certainly feels a little heavier to cast but most of that is almost certainly the heavier 9wt line.