GA, I have a 30D and a 5D and wasn't suggesting you change to Canon. I did change as Konica Minolta/Sony were not offering a body that would give what I needed.
For drag boats obviously we are talking a lot of speed which causes people to panic straight away however it will depend on how far you are from the subjects and at what angles you are shooting. If you are shooting with them heading straight towards you then AF speed is key and you might benefit from the better AF handling of a D200 but assuming you have the HSM version of the Sigma then you are probably OK. Now the question was raised earlier if the 70-200 was going to be long enough and only you can answer that.
Now due to the speed and short run times you might find that having the 5fps a realbonus asthere are going to be focus errors (all cameras get them when shooting in continuous focus mode) so getting another 2 shots every second gives abetterchance of you getting the shot you want.