Would it be beneficial buying the flasher for subsequent retunes? How does this tune perform compared to a dyno tune in terms of Performance?

This is correct, especially if you still get your car serviced by your Holden dealership. I say this as you can revert the car back to the standard tune when getting your vehicle serviced and back to custom after you pick it up. On this note, the number of times that we flash the vehicle using our software cannot be detected by the dealership. Given your vehicle is relatively standard, there isn't a huge difference between the two.