Looks like necrochasm maxes out at 356 attack.
Some guy who's rumored to work for Bungie in some way has it.
http://www.bungie.net/en/Legend/1/4611686018430455360/2305843009217958569#gear
Probably right. And I think 364 attack from the glitches free accounts will be the new level 34 value for HoW. Wonder if Vex and Necrochasm are both forever locked at those attack values.