Looking at the spec of the U1 and the mini on the Lumin site right now.
The only difference in the spec is the power supply and the higher PCM support of the U1.
It would stand to reason that those who are saying that the added aftermarket power supply would get you damn near the performance of the U1 have a pretty solid case. Yes, I have the U1 Mini with the Sbooster, and I'm very happy with the results when connected to my Bel Canto DAC.