They will stand up to it just fine.. There is a big benefit to the upside forks, you will have quite a bit more under axle height, which can increase your confidence when dealing with a really tight rut. I went over the bars once on my KDX because I snagged a rock in the side of a rut.
With inverted forks you do have to pay a bit more attention to damaging your lower legs.. Suppose you are trying to make it up a steep hill, and you can't make it and drop your bike, and you were in a bit of a rut.. Now the bike is going backwards down the hill, scrapping your lower legs on all sorts of stuff imbedded in the dirt. I'd personally recommend seal savers in addition to the regular roost blocking fork guards.
ps. The WR250F has inverted forks.. Ride one of those and then try to compare it with a KDX's forks.. There simply is no way to compare the feel of the 2, the Inverted Kayaba's are worlds better in all condtions. (except maybe in steering precision/stiffness).
Jon!