Interestingly enough, it seems to have little to do with the clock speeds. 

With dpm turned on and radeon_dpm_force_performance_level set to high (doesn't
switch properly):

uvd    vclk: 0 dclk: 0
power level 2    sclk: 75000 mclk: 80000 vddc: 1120 vddci: 0

With radeon.dpm=0 (WORKS, switches quickly):
default engine clock: 750000 kHz
current engine clock: 749980 kHz
default memory clock: 800000 kHz
current memory clock: 799870 kHz
voltage: 1120 mV
PCIE lanes: 16