That answer's wrong, JS. They run modest, you don't need the speed boost at +2 and it's not like his speed tier is relevant when nobody uses Haxorus or Hydreigon, and Kyurem-B can't touch him.
Source: Smogon irc, experience, common sense.
edit: well obviously people who blindly follow outdated Smogon analysises will be running timid, but they shouldn't and you shouldn't either ;)
edit 2: http://www.smogon.com/bw/pokemon/genesect is more up to date than the DW analysis and lists a modest nature first.