If depends on the iem in question. You'd have to know what the impedance curve of that IEM looks like to know how it will be impacted. Many dynamic driver IEMs have flat impedance curves and don't change much, if any, with added impedance. Many BA IEMs, especially multi-BA's, have wildly fluctuating impedance curves. It can cause severe treble roll off or spikes and the same for bass.
Innerfidelity has a lot of IEMs measured and includes impedance graphs as well:
Rin's blog is also helpful:
Reading an impedance curve can be confusing. Just know if you have a sensitive iem and you see a wild impedance curve, it will be affected. Sometimes the change can be nice, sometimes horrible. That's the over simplified explanation.
Edited by shotgunshane - 3/15/13 at 7:30am