As others are pointing out, we're talking about pure fat weight loss. Keep lifting the same amount of weights, increase the cardio a little, decrease the food a little.
So let's flip it around. Picture how Davis would look if he just lost 20-30 pounds of fat. Probably a little like Boozer; that's a good comparison. So, Picture that type of player: Strong, solid...now, which of those players would ever, in any circumstance, be Better if they Gained 20-30 pounds of pure fat?

NO ONE!! There are certainly players that could be better by gaining weight...like anthony randolph...but in that case they are talking about gaining weight by hitting the squat machine and building muscle mass, not just overeating.
And absolutely, the concern about Davis in the draft was about whether he'd be able to keep his weight or lose it, with losing weight representing the greatest upside. Shaq was always going to be #1 based on size and skill, but he too always played better as he lost weight.