Number of issues on purchasing ddc for I2S. Very few streamers offer I2S output so in these cases ddc required for using I2S. Another concern is quality of clocks in ddc vs dac. If dac internal clock is equal to or better than ddc clock you're not getting any benefit from clocking with ddc, only possible benefit is the I2S pathway. Greatest benefit with ddc is when it's clock superior to dac internal clock, in this case more likely to be superior to usb. Whether it superior or not depends greatly on the quality of that clock, a high quality OXCO clock with high quality power supply to it is optimal. How these clocks implemented critical, not all clocks created equal, such that a poorly implemented OXCO clock could be inferior to quality implemented TXCO or Femto clock.
And then we have concern of input to ddc, generally this going to be usb, this should also be optimized for highest performance. A noisy or poor quality input to ddc will greatly impact ultimate performance.
My point in the above is don't automatically expect I2S to exceed usb, or in some rare cases other interfaces. Many dacs have sophisticated usb boards, less than optimized I2S will likely be inferior, especially if you have usb chain already optimized. A previous foray into I2S using Singxer SU6 and generic I2S cable proved inferior to my optimized usb setup.
