I'm very late on this, but the flywheel is only a very small part of the equation. (and for 1979 model, it is quite the same as on the /5... )
The main improvement where first the gearbox input with it's spring shock absorber. Then they put another one in the swing arm and that did the trick.
On the 1981 model, they lightened the clutch and added some smoothness to the ride.
I also own a Guzzi (V7-III, E4 model) and I miss the big spring in the swing arm a lot. The bike react violently at every bump, even with the most expensive shocks and a lot of tuning...