Does anyone know if adjusting the diet of your pet actually helps? 

Really? I've got several of my pets on specialized diets and I haven't noticed any difference.

Have they leveled up?

Stats only increase when they level up, and it is based on the diet.

Yup, the diet of a pet really does affect it's stats

Requiem here is a prime example: http://beastkeeper.com/pet/125

diet only effects during level up. i have one somewhere whey i started to record the gains but then my stable made me forget which one and so i didnt get it updated ...

man i wish that i had the "pokemon" level up box thing x3

Yeah, they're leveling up. They've had stat increases, but I haven't noticed it being connected to the diet.

I believe it goes the more you allocate in a particular food, the more likely you're going to get stat increases in the stat it boosts because every point represents a chance to gain a single(?) stat. I adjust my pets' diets according to their genetics. My unicorn Moringa has a 50% boost in her strength, so as that's where it will benefit her most I've focused on boosting her strength by having 6 points in animal fats for her diet. At lvl 18 she now has 101 total strength and 67 base strength - 15 points higher than her next highest stat at its base (though she may have had a naturally high base strength as well and missed a couple of levels). On the other hand her base intelligence is 39, 10 points lower than her next lowest stat- it's the stat I focused on least with 1 point given to animal carbohydrates.

It should also become more noticable the higher the level your pet.

Thanks for the feedback guys ^^ 

Most of my pets have special diets playing on their strengths: No food (or only one or two) allocated to red stats, full points to green stats, and either even spread amongst neutral stats (or a slight boost to the higher neutral stats)