My 78 B has the original Zenith, rebuilt and running the original style air cleaner. It is desmoged and it runs great! In fact, it still has the automatic choke on the Z/S and even that works very well. It took some fiddling, but I am really pleased with the way it's running.
My mom has an 80 B with a weber side draft, headers and a free flow exhaust. It is her year-round daily driver so it's used in all conditions. The car is definitely a little quicker than my stock car, but I find the "bark of the weber" a little annoying, not to mention the headers add to the noise. It sounds great at high rpms, but at low rpms it sounds a little rough. I also find that the power delivery is not as smooth. The weber is not without it's problems either, it seems to be suseptible to heat rising up from the headers and causing vapour lock if a heat sheild is not installed and the car is run in very hot weather. We also had an issue with the carb body cracking where it mounts to the manifold - likely a result of expansion/contraction or vibration (car is driven hard and a lot!).
A few years ago we had another 80 B-LE which had a weber downdraft and headers. Very similar to the sidedraft, with a little less punch. It seemed a little less finicky though.
I've not driven a later B with SUs, so I can't really comment on that option. All in all regardless of the setup it's most important to have everything dialed in correctly. A stock B running 100% feels so much better than a hopped up B running at 95%, and a hopped up B running at 100%, while notably quicker, comes with it's own set of compromises.