The location and magnitude of that peak can potentially change each time you insert your IEMs, as well as over time while worn if they start slipping out due to ear canal movements, sweat, or wax. What sounds right to you now after EQ might sound off tomorrow or even 2 hours later. Also, good gear is generally tuned with that resonance in mind, which can cause simply chopping off parts of that peak without making other changes to compensate to throw off the treble balance and hurt timbre or imaging.

If you're not sure what you're doing, unless there's either some nasty vocal sibilance (harsh S and T consonants) or an unpleasant piercing or metallic quality to percussion instruments like snares or cymbals that makes them painful to listen to, it would be best to leave that peak alone.

Even if so, I'd suggest trying out different ear tips first to change the depth the IEMs tend to end up seated at in your canals, although do be warned that nozzle size will lead some tips to be incompatible with your IEMs. Foam or Ostry tuning tips (often listed as "turning tips") also used to be commonly recommended for attenuating excess upper treble on chi-fi.