“The real question is: where does the point of diminishing returns begin?”
At what price point? Also, “diminishing returns” is subjectively defined by each owner/listener, for their own system. IOW, not a stationary target.
That said, in my system, the $6,000 Sonore Signature Rendu SE Deluxe (streamer only) provides significantly more engaging results than the $1250 Metrum Acoustics Ambre, $7,500 Antipodes DX Gen 3, or $7,000 Mojo Audio Deja Vu that came before it. Keep in mind, the Deja Vu and Antipodes DX are server/streamers. Of those, the Deja Vu came sonically closer than the others to the sound of the Sig. Rendu SE but, even then, the Sig. Rendu SE Deluxe clearly sounds better to me. A good comparison would be against the $8,000 Innuos Pulsar (streamer only).