Yes, IEMs benefit. Something like the Etymotic ER-4S, for instance, needs an amp. The old Altec Lansing IM716 (an Etymotic OEM) could not perform well without an amp, period. Grados are known for not needing an amp, yet almost every one but the bottom-of-the-line SR60's perform better with an amp.
All this talk about impedances is too confusing. It really doesn't matter when it comes to whether you should use an amp or not. Besides, very few amps publish their output impedances. You have to find someone who's reviewed one and then went even further to measure it, before you can determine the output impedance. There seems to be a huge emphasis on amplifier output impedance lately. Maybe it's good for education, but it's not a panacea. In fact, higher damping factors (the ratio between headphone impedance and amplifier output impedance) don't necessarily translate into better sound with all headphones.
The impedance of the amplifier determines how much power it will provide to a load at a given impedance. A high impedance headphone will need more voltage swing than current impulse, whereas a low impedance headphone will need more current impulse rather than voltage swing. Portable devices are limited by batteries, for instance, which means their voltage swing is limited. That's why you may see many recommendations for low-impedance headphones to be used with portable devices. On the other hand, high impedance phones need a high voltage swing but little current. Portables can't provide that because of their battery limits.
That's almost a digression, though, at this point. What's easier to consider is POWER of an amp and EFFICIENCY of the headphone. Power is the end result desired, period. If you don't have enough, the impedances in question matter little. When it comes to power for instance, just to hear a 3dB difference - usually the threshold of where a human can hear a difference in sound volume - you need 2X power. For a 6dB difference, you need 4X power, but the real relationship is exponential. If you need a 12dB peak, the power requirement is 15.8X power over 0dB, and so on. (Even the worst compressed music usually has at least a 20dB range on a peak level meter.) Pretty soon, if you consider fleeting transients and momentary peaks, you find out that power needs increase drastically very quickly.
The main point is, every device that powers a headphone contains an amplifier. The real question: is it sufficient? If you think you're listening to a headphone without an amp - you're not. So are you missing something, confident that whatever amp is contained in whatever device you are using is sufficient to capture every detail, every transient, every musical peak without clipping into distortion that you typically never notice because you've heard nothing better?
That's the question you have to ask yourself.
