You climb with a climb speed schedule 300/.78. What do you expect in the crossover altitude 29 200 ft (OAT = ISA)?
During the acceleration to the Mach number .78 the rate of climb is approximately zero
The rate of climb decreases since climb performance at a constant Mach number is grossly reduced as compared to constant IAS
The rate of climb increases since the constant IAS-climb is replaced by the constant Mach-climb
No noticeable effect since the true airspeed at 300 kt IAS and .78 Mach are the same (at ISA temperature TAS=460 kt)