Have you read my thread "BA XR8 Performance Fluctuation". Even after another computer, and complete throttle system, I still get a variation from time to time. I have heard of another guy in Victoria who was having similar problems. Sometimes on top of the world, other times "what happened to all of the grunt", But my interstate contact said he had disappeared and not returned to the dealership (maybe fed up). From your descripton I don't believe it has anything to do with traction control. I got frustrated when people told me it was that, or "you're just getting used to the power". Because if that was the case i would be used to it all of the time and not notice when it goes hard.
'03 BA XR8 Manual, Edit, Hi Flow Cats, HPX twin system, King Springs, Battens C.A.I.