Can't speak for the T5p2, but between AKT8ieMKII (which imo has similar sound to Xelento) and T1, the difference is in the bass and treble. Mids have similar tuning imo.
Bass on T1 is tight and high quality. You can hear every bass note, every shift between one note to the next. On the T8II, midbass and upperbass is slightly emphasised, so you get plenty of bass quantity especially for EDM, modern pop, which makes it a really fun headphone. The bass pump during a Skrillex playlist is something to die for. However, for the T1, I find it less suitable for modern pop / billboard top 50 kind of songs because first they aren't always recorded / mixed with at the best quality, so garbage in garbage out happens. The lack of bass quantity in the T1 also makes modern pop sounds rather flat because they rely heavily on bass beats as the main body of the song. You get a clean sound from the T1 that's for sure, but whether it's enjoyable, then YMMV. T1 bass is very good for concertos, orchestras, percussion, etc as that is the intended use case.
Treble wise, T1 has some peaks which applies well for wind instruments and cymbals etc found in percussion, orchestras etc. But I find that it can be a bit sibilant for female vocals on less well recorded tracks. T8II also has good treble extension but the bass and mids are so pronounced that treble tends to get drowned out, or in other words, less emphasised. They are present but not thrown in your face, so you have look out for the details.
Soundstage wise, I am 100% sure T1 has a much better 3D soundstaging and instrumental layering thanks to its bigger, angled drivers. That's something that so far in my experience, only the HD800 can rival. T8II's soundstage is quite good for a closed back IEM but is deeper than it is wide, thanks to its excellent bass response. My rough estimate is maybe 3 meters radius around you.
Overall, T1 is much better for classic, orchestra, "audiophile" grade type of tracks, but to me it fails to impress as a modern pop music headphone or any bass-heavy content. T8 on the other hand is amazing in those modern pop genres but can get a bit cramped for genres requiring absolute sound staging and layering. As such, they complement each other and which you pick depends on your music preferences and priorities.
For me, as a frequent traveller and commuter, portability is king, so I chose T8 three years ago, and I'd do it all over again. IE800 and Katana would be my alternatives. And now that true wireless headphones are out, I chose the Momentum True Wireless. Amazing SQ in such a tiny package and half the price of the T8. Did I mention it is wireless? But that's a story for another day.
