I should add that a lot of this is a question of tuning.
A well tuned set of sidedraft webers is hard to beat, but tuning them can be a very frustrating, expensive, and time consuming proposition. The number of variables that makes them tunable over a wide range, also makes systematic tuning nearly impossible as there are too many paths to go down with different series of emulsion tubes, etc. For the typical '50s, '60s, '70s useage where all we were looking for was throttle response and power, this wasn't a big problem. For race engines where the only criteria is throttle response and power from 4,000 - 8,000 rpm they are easy to tune. But start adding fuel economy and smooth light throttle response from 1000 rpm, with a stock exhaust that you can't radically change to help in the tuning process and it gets a little complicated. Phil is one of the few who seems to be able to get this right on a B20.

Try emailing weber tuning instructions to a novice weber owner and you will know what I mean right away. And then tell him what it will cost to try a different set of emulsion tubes with main and air correction jets - approx $96 at list prices for a pair of 45 DCOE's. When I go to a dyno tuning session for webers I take 4 plastic cases that have several thousand dollars in jets, chokes, venturies, emulsion tubes, etc.

I might add that as part of this process I also experimented with Weber's newest carb, that was designed to be an SU replacement. It is essentially a single barrell sidedraft DCOE but with the later style diaphram accelerator pump.
It worked, gave better high rpm power than HS6s on the same engine, but there were problems with them that made them hard to adapt to this application. They were replaced by the Mikuni in testing.

The Mikuni flat slide is much simpler, a lot easier to adpat as an SU replacement in a Volvo, and tuning for various throttle/rpm/load ranges is much more direct. And the number of parts needed for tuning is probably 1/10th both in terms of number and cost. At this point, after 20 years with sidedraft webers and a little less than a year with the Mikuni, I'm convinced that the Mikuni is the way to go. But what I feel about it doesn't mean anything.
The performance numbers will decide it.

John
V-performance.com