First off, that is an AWESOME deal!!!!! I have paid more than that for Rs that were parts bikes. If you haven't rode one much, be careful the first few times. They can be a handful in the woods and on the rocks if you are not used to one.
On the speed thing, I am gonna say it depends on where you race at. I am not sure about a stock R since it has been so long since mine was, but I think that my big 3 XX would destroy mine on the road or dirt. At the same time the R would eat it up in the sand or anything really loose, the weight difference is too much to overcome in those situations. Watch the front end, coming up and going sideways when you have a traction differential on the rear end. If you are going to trail ride it, I would recommend spacers and a harder rear tire like a scorpion. Makes it much easier to get around the corners.
You will love it!!!!