nikon 800mm decision

Well, Nikon is going to finally release the 800 mm lens in April so i was going to pre order one until i saw the 17,800.00 price!! As everyone knows you always need to get a little closer to get that shot. Well I am looking at other ways to accomplish this. Looking at Nikon's 600 mm (i have all the TCs, or sigma 300 - 800 or the sigma 800. I cant find any good comparisons any where as for sharpness ect. Has anyone out there used these and what are your thoughts. All my other gear is nikon, D800 E, D800, D 3, including largest lens being Nikon's 400mm f2.8 Thanks