Is that $500 off MSRP or $500 off the sticker price for the Demo? I think since those 260 miles are sorta rough (no one pays attention to rev limits etc on a Demo car) and its pretty close to the end of the model year, you should get more then $500 off MSRP for a demo car, especially since its easy to just go to somewhere like Mini of the Hamptons who probably also have similar (though not exact) cars in stock that aren't demos.
Besides paying less (maybe you could get free floormats or something else useful (bonnet stripes if you want them, etc)) it seems like a good idea. White roofs are better in my opinion anyway, its not obvious the roof's a different color in black, it just looks like the car's only painted on the bottom, sorta defeats the purpose.
Let us know what happens..
Magic 8-ball was correct. All be in awe of Magic 8 ball.