Lumin U1 vs Lumin U1 mini + sbooster?

I would agree with that Dealer.  But the sbooster is big, ugly and your Lumin will require internal modifications that void your warranty.  Your choice.  Contact me if you want either

Have you tried any other streamers like SOTM 200 ultra?

I guess for me, going from my gaming PC direct to my dac vs any kind of streamer would make more of a difference because a mac mini is likely inherently much less noisy than a gaming pc.
But interesting findings indeed.  Im actually not all that surprised by your findings with the sbooster. Looking under the hood it seems the u1 mini has a pretty solid isolated SMPS.

I am surprised by your findings. There is simply no way that a Mac Mini will outperform a dedicated steamer.  Something else must have been at work here