The newest Bugatti is a convertible called the Mistral- it has over 1500 hp and the top speed is limited to 261 mph. My question is, what happens when you stick your hand outside the car at that speed?
You mean “The Bowling Ball Formerly Known as Your Head”?
I’m guessing that at top speed, opening the window would cause HUGE buffeting and instability. So let’s ignore that part of things.
Here’s what it looks like in a plane at 200 MPH. Not too bad, but probably not a lot of buffeting that would make things move more violently.
With the top down, you don’t have to roll down the window.
It doesn’t come with a top 
It’s not a convertible, it’s a roadster. I can’t find any pictures with the door windows lowered, or anything pictures of the window controls; maybe they don’t go down?
Either way, just putting your hand up would suffice for your question, and I suspect it wouldn’t be too different from the video of the plane at 200 MPH. I’m sure it wouldn’t be comfortable or sustainable for long.