Totally depends on the ear/headphones you use, how good your hearing is and what volume level you're familiar with.
Give you two examples which is my experience using both capped & uncapped A105, ZX507, WM1AM2 (went straight for Japanese import with A307).
Using 3.5mm SE connections
Shure SE846 8 ohm impedance
These must be one of the easiest IEM's to drive, so sensitive you can kill your ears if not careful when using on a laptop where only about 8/100 is loud enough!
With capped Sony DAP's I found myself needing anywhere between 80-90/120 while with the uncapped DAP's 50-55/120
Sony IER-M9 20 ohm impedance, but considered more demanding to drive
These are rated at 20 ohm but seem to need a bit of power, on capped its 100+/120 while on uncapped 75-85/120
I don't now use/have any headphones, but this leads me to think most IEM's should still be driven loud enough for most people even on the capped version, but many headphones won't be.
I just didn't want to pay more money for a crippled EU/UK (US/Canada/Singapore too with new ones) player when often you can import cheaper anyway and only potential problem is any warranty issue. My experience is resale is easier on imported versions too.