In the thread that Sourav linked to James also states (in relation to asynchronous sources like USB):
Unless I am completely misreading that he’s saying that a clock does make a positive difference when just using the DAC. And this matches my experience as well.
You may well be right that an Upsampler gives a bigger improvement if you’re limited to a two box solution but I don’t think it is right to say that a clock makes no difference.
Anyway, we can only hope that we will both be in the fortunate position to have all three boxes at some point!