DX vs FX for motorsport isn't much of an issue if you can get close (ie trackside), maybe more so if you are further back.
On your second question, not a lot of advantages of a D3 vs a gripped D700, other than the grip and batteries are an extra cost.
In fact, the reverse might be true because a D700 has a pop up flash (either as an emergency or as a flash commander) and also has sensor cleaning - which after a D300, you will miss for sure. A D3S obviously is corrects the latter point.
My colleague uses both a D700 and a D300 and when you can get close enough the D700 makes a nicer image certainly. He mainly uses the D700 with short lenses though (or at least at short ranges - ie 70-200 for grid and pitlane shots), but has shot stuff with a 500 f4 on the D700 too. I asked him a while back if he'd get rid of the D300 (or future DX body) and he said not... probably nuff said.