VMO (SL - 8000') = 300 KIAS
VMO (8000' - 30,267') = 340 KIAS
Is that a step-change at 8000'? Must be a right PITA in the descent! The CJ2 has a similar step-change at about 6000' IIRC and it was always a challenge to remember to slow down enough to stop the overspeed klaxon sounding.