I had a similar issue with my baseline, 280-ish. Even with the fuses pulled, the car still let off at around 5200rpm... After much head scratching, we discovered it was the speed governor that was stopping us.

Spoke with EC about this and they said when establishing baseline on the xf/slk/etc they make a 3rd gear run versus 4th as the numbers are nearly identical.

Fwiw, of course...