I agree, heavy flywheels have their place when you need a harder hit off the line and the extra mass will help with balancing since single cylinder balancing is near impossible since you got not only the rotating assembly but also the piston acceleration that cant be cancelled out, but by adding more mass to the crank assembly it offsets the piston acceleration reducing the effect it has on the crank assembly. So besides offering better off the line performance it really helps smooth out the engine which can also allow a higher Max rpm and top speed but at a cost of slightly less acceleration.
Some larger V twin motorcycle engines can have flywheels weighing as much as 65lb. The Honda Vtx 1300 and 1800 engines have crank flywheels that weigh this much and it definitely contributes to a smoother ride. Harley cranks are also very heavy for the same reasons and these engines not only produce massive torque but its a torque you can definitely feel in any gear. In cases like the V twin engines and single cylinder engines the benefits of the heavier flywheels definitely outweigh the cons of having a lot of rotating mass and the engines do perform better because of it so yes... this can be considered a performance mod.... as little as one ounce just 4 inches off center can apply as much as 400 lb at 5000 rpm so the heavier but well balanced engine will even accelerate better than the lighter crank assembly than can only be balanced statically. If one was to add let's say a 5lb flywheel to onenof these engines and balance it as best as possible it will accelerate just as well if not better than the lighter engine that's also balanced as best as possible for a single cylinder or 45 degree v twin as well as having a higher rpm potential and top speed. Doing the same on an engine that's already well balanced by nature like an opposed twin or inline 6 or even an opposed 12 cylinder, the extra weight would just get in the way unless someone needed it for the extra inertia for better off the line starts.
