"I wonder how much ram-effect was gained through centrifugal force accelerating the intake air in the round plenum and 'down' those intake pipes?"
Your assesment of none is probably right, but not due to centrifugal effect. The air followed a tortured path through the center of the crankshaft to get to the crankcase, then out the pipes to the heads. Yes, the crankshaft was hollow and attached to the fire wall What passed for a carbie was attached to the crankshaft
At least it wasn't a monosaupe engine that had the weird centrifugal valve in the piston crown to duct the mixture into the cylinder
The rotaries are so fascinating, so much dead end technology. But absolute works of art in execution and in their day, they were the king of power to weight ratio and reliability in aircraft engines.
Their reign was short, roughly 1910 to about 1916 in terms of the best aircraft engines available. By 1916 they were obsolete but were being made in large numbers on both sides of WWI and stayed in front line service to the end. After 1918, they disappeared rapidly.
But they are still works of art.