So finally got to fit and test the 300mm discs.
They drag the car down roughly a second slower.
240mm 9,8s 9,8s 9,5s
300mm 10,8s 10,8s 10,3s
Used the same rims, same bit of road (a slight incline), same timing method (a passenger with a stopwatch). I know there are better methods and more runs...