If a 2 stroke delivers more torque - why do they need to rev higher to produce the requisite horsepower to suit the engine load (created by the propeller)?
In most 2 strokes the torque is going down well before it reach's peak HP. The four stroke is limited in top RPM because of valve float, and no company as yet has tried the stiffer springs, roller tappets, overhead cam, etc, so that they will reach the same RPM. At the lower RPM many of the two stroke will attain a higher torque than an equivelant displacement four stroke.
BTW I think Hobbsy's formula is an approximation as it doesn't take in account all of the possible rod, crank, stroke and length combinations. Also I think that displacement is for one power stroke. For a four stroke the power stroke is followed with a no power intake stroke so the torque measured by a dynometer will be about half of his calculated figure.