I would do it by having the GV for speed set to self+random(x,x) and then with the on created disc event have the speed initially as (50,80)+GV where GV is initially 0 but then increasing to a random amount on every number of discs destroyed. That way also if you want to have different speeds for different discs or for example, destroying 10 large discs only increases the speed for large discs, then you can have seperate GVs, so speed is (50,80)+GV, (60,90)+GV2.
To break it down....
Created LG disc at speed (50,80)+GVspeed
On (GV)LGdestroyed=20, set GVspeed to self+random(5,20)
Now when a new LG disc is created, speed is set at random(50,80)+random(5,20)