Okay after one whole night of Nagi vs Yo, I'll try to give a simple and less emotional impression.
Out of the box, sounds very similar. Will be hard to tell the difference without AB-ing.
Requires very less burn in. initially the treble was a bit grainy and blur but within no time it should sound just like intended
I'll be comparing Yo and Nagi with both stock 07 tips with my source Aune s6>AudioGd NFB1
Imaging
-It has a more intimate/engaging/in your face imaging compared to Nagi. I've tested with some songs and found that Yo's soundstage can go far if it wants to( basically purposely giving it songs with far instruments) but most of the time, it's intimate presentation will force a lot of instruments forward compared to Nagi. I do still believe that Nagi's soundstage is wider than Yo. I personally do feel that Yo has a better left right sense and can feel very nice on panning instruments, making it fun on a lot of songs. Nagi on the other hand feels quite a lot more open. If you do agree with my findings you will probably find yourself spending a lot of time changing back and forth trying to figure out which one you prefer.
Treble
-No, I do not find Kira's sparkle here albeit online sources saying its a Kira Go baby. Yo definitely has a lot of treble, and don't worry it'll feel just at home if you liked kira or Nagi. It's not as sibilant and peaky as Nagi's treble so it does make it a tad bit more comfortable. Don't get me wrong, the treble is good but it doesn't carry kira's sparkle or Nagi's extended thin sibilant treble.
Bass
-Out of the box you will probably feel that the Yo has a bit more punch on the bass. Its not a huge addition that makes it bloaty, it just feels like a bit more punch compared to Nagi. Like Kira and Nagi, Yo has less sub. After AB-ing, Yo seems to have a bit more sub bass compared to Nagi but that could be what i feel due to the closer imaging on Yo
Conclusion
-Slightly less treble and slightly punchier bass compared to Nagi. I personally feel that the main thing to focus on is the different style of imaging and intimacy that Yo provides. Personally it depends on the songs. There are some songs which sound better open with Nagi and there's some songs that sound good up in your face. I had an incredible night of stuffing Yo into my face. The intimacy makes it engaging, immersive and if i can say, emotional on some songs. And no i do not feel any driver flex. Due to the different imaging style they have, i am not able to confidently say which has better seperation but probably Yo.