Without researching all the physics behind it, I reckon 5mm extra stroke will make a significant difference. I know it doesn't sound like much, but picture the thing turning 50 times every second (that's only 3000rpm) and you'll start to get an idea of the increased inertia caused by moving the crank journal and rod big end 5mm frurther away from the crank centreline. Piston speeds are increased dramatically travelling 10mm further for each stroke (ie 20mm further for each crankshaft cycle), and rod angularity is increased placing greater loads on it and the piston skirt. Although this may be offset by using a longer rod, I suspect the A15 rod is more than likely shorter to allow for the increased stroke and to improve torque. If my tenuous grasp of physics is correct, shorter rods will increase the max piston speed as well.