So been trying all the different modes on the 9038s. I can confirm the IEM mode is the least detailed out of the 4. It sounds a bit muffled and slightly congested in the soundstage compared to normal mode. It has even been discussed by many user on the 9038s discord pages. Performance mode is normal mode on steroids, more clarity, more detail, a bit more low end, a bit more treble extension while being smoother, more soundstage, more and improved of everything but it's not a huge difference, but is noticeable. SE Tube mode is great for brighter gear, pairs nicely with the T800 when you want to have a relaxed and long listening session.
At the end of the day, it's about what you truly enjoy most. Just trying to make listening to music a bit more enjoyable haha, try out all the modes and see which suit you ears and the T800 best for you. For me, Mode 2 is my go to
My main concern is the new cable 171 8 wire that I am waiting for. It has been stuck on Shezhen airline for nearly a week now and nothing has changed -.- . I just want my cable haha