Flywheel/clutch/crank/etc mass stores energy. This can be an advantage, or a liability, depending on the situation. While more mass does slow the rate of change, it also provides inertia to dampen engine pulses and provide "momentum" to overcome variations in applied load (minor ups and downs in a roadway for example). More mass will give a smoother idle and make the vehicle easier to start moving from rest.
I have built semi-race engines (if your engine idles at all, it's not full-race) for the street. You get a high, lumpy, uncertain idle that requires frequent throttle inputs to keep running. Starting off from rest is more akin to launching with high throttle inputs and careful use of the clutch. The vehicle constantly changes speed as the road surface fluctuates. All this is extremely annoying in busy traffic requiring significant concentration to the engine's continued running that would be better put to minding the surrounding traffic. It also requires the driver act with a significant level of precision, especially when cornering near the limit. Clumsy, or careless, inputs results in the vehicle jerking or pogo-ing down the road.
In counter-point, you get a lively, responsive engine that responds instantly to your inputs. This can be quite intoxicating and gives a strong feeling being one with the vehicle.
When I built my Europa, I knew exactly what I wanted. I knew that, on the street, a bit more torque in the mid-range is far more useful than a few more hp at 7K. Higher compression, medium ports with smooth transitions, higher lift with moderate duration and 40 rather than 45 carbs. I'm very happy with the result. I get a pretty reasonable idle, easy launching*, plenty of mid-range torque combined with a nice high end hit at 6K. My flywheel is the slightly larger 215mm with no weight reduction.
* a good portion of which was fitting a hydraulic clutch -- light and smooth as butter.
