Can't really peg any of those as XC bikes, but it is the type of bike you'll be able to ride almost anywhere. Whether on our local XC trails or out in the mtns, the Yelli is great.

I can't speak on the other bikes, since I don't have any direct experience with them, but some setup issues can come with the short chainstays. If you want to run wide tires, you might have a problem with front derailleur setup. Chainring size(in the middle ring position of a 3x crank) can also be an issue. That's a 32t Raceface Narrow-wide ring in the middle position, in the photo. The tooth profile of the RF NW does make it closer to the chainstay than a standard geared ring in the same size, so you can run a slightly larger geared ring.

If you have the stock spec(Bikepedia - 2011 GT Karokoram 1.0), you may need a new bottom bracket depending on the width of your current BB shell on the GT -- width isn't shown on that link. May also need a different size seatpost, size also not listed. Other than those and the headset, you should be good.