Have you seen the movie, or are you just going off of what you've read in the press? I haven't seen the original cut of Brown Bunny, but the one that was released on DVD is definitely worth watching. It is by no means a "hard core sex scene" as some reviewers have stated. What is actually taking place is anything but erotic.
The prestige of the festival is nothing for gauging what they will show. I've seen some of the worst movies I've ever seen in my life at Sundance.
Best of luck on entering your feature. The only way for people to see your movie is to get it out there, and to as many hands as you can! What's it about?
As a side note, here's what Ebert who famously called The Brown Bunny the worst movie in the history of Cannes has to say about it now:
http://rogerebert.suntimes.com/apps/pbcs.d.../409020301/1023