Nowadays the IAS on the PFD is not the raw meassured AS. It is altered with internal logics by the ADCs to have a more accurated indication. So it is close to CAS.
If the speed mentioned for the safety valve is CAS, then it is close to the IAS. Then it is safe to say that when climbing through the higher FLs with a constant Mach. The IAS will end up well below the 280 or 260 kts.
Is this safety feature then not applicable up there?