you would have to turbo the xr6, if you got a sprint to get 250fwkw it would not be as hard as they came out with a fiar bit of power. a GT40 manifold, unichip, extractors, exhust ( which you would probably need to run a turbo anyway ) and some head work, or better yet after market heads or GT40 ones will get you close to 250fwkw. I got 161 rwkw out of my EB XR8 with that combination except for the heads which is close to 250 fwkw. now i want 250 rwkw though thats why i am going stroker.
hope this helps