PostmanPot
Honorary Master
Obviously my complete lack of interest in gaming is showing here. I still don't see the need for it unless you have polyphony issues in games...
Here are Sennheiser HD600's, audiophile/music headphones, with impedance of 300Ohm - http://www.sennheiserusa.com/over-ear-headphones-highest-quality-headphones_004465.
So assuming you are aware of high impedance headphones, not sure why you don't see why headsets targeted at gamers should not need to be high impedance?
Of course you could also try a Logitech G35/Roccat Kave/Steel Series Siberia/Razer Banshee yourself, and compare them to something requiring amplification, like the PC350's. As others have done.
Or look at the specs...