Here is a great article from WebBikeWorld on the ECE standard
http://www.webbikeworld.com/motorcycle-helmets/ece-22-05.htm Couple of items to note from the article:
"There have been several studies of motorcycle crashes over the last 25 years that have attempted to evaluate any protective advantage or disadvantage of helmets meeting one standard or another (Hurt, 1981; tte, 1991). No advantage has ever been shown in these field studies for any particular standard, so the helmet industry and individual riders are left comparing theoretical pros and cons of the various standards."
"Helmets certified to the ECE 22.05 standard are approved for competition events by AMA, CCS, FIM, Formula-USA and WERA and are chosen by nearly every professional motorcycle racers competing in world championship road racing, motocross and off road events, including the ultimate sport of Moto GP."
There are modular helmets that meet bot ECE & DOT standards, such as the Schuberth C3 ( and many others).