Having chopped and changed rear shocks such running a suzuki bandit 6 with honda 600rr rear shock ( I was the first i believe) I wouldn't worry about it too much other than getting the static sag right and springs that fit you wieght kitted up. Hagon will sell you springs of a different rate.
The angle of the shock also makes a big difference, not sure which Guzzi you've got but I don't think they'll be too far off.