Certainly makes it more convenient when you have to pull more than 9.9 degrees of timing. Also has lots of nice other features you can use on less than marginal tracks, as well as several functions making it possible to go back and look at the run as well as monitoring vacuum. Which is always nice on a nitrous motor.
Now that I have a 7531, I often wonder what I did without it.
For us, the biggest factor to get us over to the 7531 Box was traction (or loss of) on slick tracks in 100* heat & 140* track temps, and on a small tire with a heavy car. Instead of fighting with smaller multiple stages & progressive controllers, we use the Timing Retard & Launch Retard & Ramp functions (Gear Retard also available) to leave with less power, but can bring the power back in as the track permits.
Like x289sec said, I now wonder why it took me so long to go to the 7531.
As far as amount of power you need before switching, I don't really think there is an answer like that. As you can see from the posts above, many people have gone plenty fast with a Digital 6. It's just when you start to need the features offered by the Digital 7, that it starts to look better.