The most effective and accurate way to answer your question is: shift yourself, because D and DS will both take you to 7000rpm at WOT.
If you have a tune, log wide open throttle (WOT) runs, look at your boost curve on your data log and find the point in RPM where your boost starts to drop off to a psi level that is lower than where it would be at the start of the next gear. Ideally, you want to shift so as to maximize the area under your power curve.
The simple answer: at WOT shift around 6200-6300rpm.
This all is assuming that you have an n54 thats tuned. I'm on the mobile and can't see what you drive.