The sand dunes are not somewere to be just ripping around and going all out. It is VERY dangerous to say the least. In the Imperial sand dunes life flights are going in and out 24/7 no joke. If you stay smart it will be the best ride of your life without a doubt but it you get stupid it WILL eat you alive and put you on the helli on your way to the nearest hospital. Racing is a cut throat world. Nobody (while on the track) is going to be nice. If your in there way and they want by your either gonna move or be laying on the other side of a berm. If your an experienced rider hit it all out and don't be afraid of banging some bars by all means. Sounds like you've had enough seat time and the 250 is a great race platform.
When I go to the dunes I go to the little sarah where it is not near as busy. there a miles and miles of open desert with no one in sight. wich i guess that is kinda a recipe for unaware disaster and plus I'd be riding along with a rzr 4 and a couple other ATV riders.
I've never raced, but I do ride dunes. All I can say is ALWAYS make sure you know what is on the other side of the hill. Otherwise you will be in a world of hurt. Also, the sand is a MoFo on your clutch. Be sure it's strong before you go, or you will find out fairly quick that it's not fun watching everybody ride while you look at your quad holding down the fort, lol. Respect the sand, and it will respect you. Other than that, it's the best time you will have in your life!!!