Couple things:
Bilstein has digressive valving, which has a certain 'feel' that isn't always the best for off-road in some circumstances. The 'big boy' coilovers will have more linear or progressive valving that allows it to be more plush and still control large suspension events effectively at the cost of 'sharper' handling when you need faster transitions. Read up on valving styles to see what will work best for you.
What tires (size, brand/model and load rating) are you running and at what pressure?
All that said, I run stock front springs, OME2906 rear springs and 5100's all around and it all seems to perform decently at high speeds (stage rally course opening car, up to 70mph on dirt) with 20-22psi in the tires, though I KNOW it would be better to be on a set of Kings, etc.
-Charlie