When I ran the worlds in Kinabalu in 1999 a lot of the teams turned up with racing flats and then checked out the course and had to get studs flown in...!
Basically in this competition it depends on where the race is and whether it is the uphill only year. This is not normal trail running either as the speeds that the top athletes can get up to during decents is close to 100m sprinters. A lot of the euro courses a dry dusty trails and hence flats are the best shoes especially when it is uphill only.
You need something low profile with soft lugs. Most of the trail shoes have to sacrifice the soft rubber in production for more durable hard compounds. The specialist companies like walsh and innov8 can afford to be a little more extreme.
Generally the mainland europeans wear flats, the commenwealth countries wear studs (walsh, innov8, adidas swoops, Not sure what the yanks wear but i think they wear more adventure style racing shoes.
My personal choice would be the old adidas lightfoot, followed by the classic walsh PB's. Super light, super grip, low profile.
If you run on soft trails then you don't need cushioning and a pair of studs are good for runs over 2 hrs. Another option is to pick your fave racing flats and send them to Pete Bland Sports to get them resoled with some aggressive lugs.