The D800 is probably a better solution anyway, particularly the E, but I can live with 12 bit for motor sports. This also shows what a versatile camera the D800 is. Although not perfect for action, it will easily follow the D300s in DX mode, and that camera is really the only current alternative south of $5,000.
I haven t shot motor sports but in the winter I shot Polo in Florida . Polo has been my testing environment for my Nikon s and long lenses . Others uses Surfing ,Tennis etc aren t nearly as demanding .
For Polo the lens to have is the 600/4 ..a beast but on a Monopod its very easy to balance . Fortunately I can drive to within 100Ft of where I might shoot . This isn t a carry all day situation . Panning with a 600 is a bit much and here a 300/2.8 is a better choice .
I have used the D3,D3s,D3x and the D800E . This is a situation where when the horses charge the goal ..you lock on the ball and shoot 8-12 captures . By far the best alternative has been the D3S (haven t used the D4 yet ) . The responsiveness of the D3S is just better and the IQ is excellent . The color much improved over the D3 ..not as good as the D3x or the D800E .
The advantage of the D800E has been the great files ...so if you intend to work the images ..you have plenty of room for processing . With the D3S ..you want the file to be pretty great right out of the camera ...not much room to crop for example .
I ve shot with the guys that shoot for Sports Illustrated and I noticed that some of them edit constantly in the camera . The review each series and lock the keepers then delete everything else . This cuts down considerably the edit time when you shot in series . They assume the file is done when they turn in the card .