I like grivory, I like micarta, I kinda like the factory sheath, but I like others better.
For the total package, get a sheath that will work for your needs. Do you want a knife kit? Just a sheath?
For a performance upgrade, get you some micarta. It dresses the knife up nice, gives you more grip (maybe), and absorbs shock better than grivory.
That said, here's what I would do, mod the factory sheath by filing the inside with a mill file, so the edge won't drag and drill out the rivet closest to the top of the sheath. Either go with that, or get you a Tekloc, leather, kydex or something to reinforce or change the belt loop. Leave the grivory alone, its perfect. If you find it too small, ranger band that bad boy.
Buy a BK11 or BK14.
Moose