You can't compare headphones to IEMs. The 2 trandsucers have their pros and cons, generally headphones have superior soundstage, whereas IEMs are more for their portability and perhaps isolation (though they generally have in your head soundstage). So they are complementary rather than competing.
From the midFI IEMs ($100 - 500 USD) region to > $1K IEMs, there are already huge diminishing returns. You are paying 10x more for perhaps 20% improvement in technicalities. From $2000 -> $5000 USD, the differences are even more marginal. Even TOTL IEMs have weaknesses, and price does not correlate with enjoyability or even sonic fidelity. I would strongly suggest for gear > $500, best to not do a blind purchase and best that you go down to a shop to audition it first, or perhaps some online shops with a returns policy. Would be a mega white elephant if you spring a purchase of a few thousand bucks and not like the sound or fitting.