Developers always drop this lame old line about how the demo / beta / leaked alpha "doesn't represent the final performance or quality".
Bull. I've NEVER seen a bad demo that turned into a good game, nor have I seen performance increase by more than a few percent in the final version.
Here's a tip. If you find yourself apologizing for the quality of your demo, just don't bother. You're better off trying to go for the extra sales to suckers who buy games based on the box art, rather than offering them the chance to see how bad your game is before they've already paid for it.