I used ibasso DX90 and mainly Jazz tracks from various artists. For bass track, I usually stick to Pink Floyd's DSOTM. Just like you, I never believe in burning in for IEM, speakers and even cables. None of these are scientifically proven. The reason people felt the improvement over time is because their ears got used to the new sound signature.
Before GR07 BE, I was using hifiman re400. If you are familiar with re400, you will know it has a more lush and mid centric signature. When I changed to BE, I felt the vocal taking a slightly more recessed presentation. Didn't like it initially but got used to it and started to appreciate the details it brought.
Changing from BE to P1 was easier because the difference was not as pronounce as re400 and I was in need of a better build IEM. I took the plunge without extensive testing. With both IEMs in my possession, it was easy to run a AB test.
P1 to me has slightly more finesse and smoother in almost everything it does as compared to BE. In fact,the difference was so subtle that unless you are critically focusing on micro details, you would probably have missed it.
Testing GR07 X in the shop was almost the same experience as when I was testing P1. In other words, everything was just marginally better than BE but still felt P1 was better based on memory. But like I said, price being almost equal between X and P1, I will take P1 any day for its construction and build quality.
No blind test was conducted so do take my words with a pinch of salt. I urge you to try P1 on before committing.