BN2A
In VNAV mode there is a short lag as the schedule changes from CAS to Mach number. This results in a slight reduction in pitch and consequent reduction in ROC. As you point out the change to constant mach number will result in a increase in ROC although climbing through the 30's the gradual reduction in performance tends to mask the difference.
The point I was making was that if left alone or a temporary use of the speed mode is utilized the aircraft will either resume its previous ROC ( or close to it) or if speed mode is utilized the reduction in ROC can often be avoided. It is very common to see climb 1 selected due to the VNAV pitch/ ROC reduction at FL290/300 when it is not really necessary.