So I waited until I got the optical cable to give some more feedback on my experience with Chromecast Audio. In the meantime I let the Modi to soak in and also got the cable adapter to connect my iPhone to the USB port on the Modi. Streaming the same song using Tidal HiFi, the sound quality using the iPhone -> USB is clearly better than the Chromecast using the optical output and significantly better than the Chromecast analog output. The analog output is noticeably more veiled in comparison. I’m of course using different cables for each scenario but they are all moderately priced so that should not be a factor. The optical and USB ports on the Modi also appear to have similar specs. Anyway, I still say its worth the $35 selling price just for the convenience it adds to stream wirelessly but in my set up and to my ears, the phone/tablet -> USB connection is far superior.
I was tempted to take it downstairs and do a comparison with my Bluesound Node 2 in my main system but I figured it would ultimately be pointless since there are too many differences between the two set ups to draw any meaningful conclusions. Everything from a dedicated 20 amp circuit and power conditioner to cables and speakers and everything in between is different.