Adding width to the base makes a fin faster ?
Yes, if the foil is better at "shepherding" the flows. Notably at the tip and especially at the base, where we are not in the "infinite foil" theoretical case and a lot of weird real-life effects happen by the interactions with the board.
A well-foiled foil will be faster (especially at the slow speeds of surfing) than much thinner square planks. And surf better.
I have only used Quobba fins since discovering them more than 2 years ago. On slow weak waves they do not make a lot of difference, but once you experience the added hold and speed in power, you get instantly addicted. I have used a lot of fins in decades, and they are the only ones that really changed my surfing, as the speed ceiling and hold of my boards have been raised it opens up new parts on the wave. And this even when they had only sizes M for my 100kg (I use XL fins normally).
Note: As an engineer, I was very skeptical when I just looked at their published "science" as it seemed quite bogus (to put it nicely). But I am a fin addict and cannot resist trying things, especially since the classical parts of their foil seemed well designed, unlike a lot of big brands. So I just ordered a set. First session was meh. On the second one, a nice little Hossegor fast peeler changed everything.