Paul, with great respect, anecdotal evidence is not worth much. Volvo decided the B20 was a worthwhile improvement, in fact they planned for 2 liters when the B18 was first cast.
My engine building experience is not limited to Volvo engines. Displacement is always a means to increasing torque, which is what increases horsepower.
Obviously the B18 can be made into a powerful engine. The B20 can be made more powerful. Volvo R-sport bored them out even bigger - for more power.
It's the North American way - bigger is better. I built a 427 Ford Cobra replica a few years back. I thought the 427 was a bit small, and very expensive. I built a 460 cubic inch engine, when others said it would be a waste of time. Guess what - I beat a 300 pound lighter, 427 powered replica that everyone thought was the big dog on the block - until it met my 460. Yep, anecdotal I know.
I'd still go with the B20, which until 1974 had essentially the same rods as the B18.