Stuffing yourself to bursting is no kind of preparation for a fight, but in the Chinese Classic "Outlaws of the Marsh", characters routinely do this we beef and wine before leveling opponents. In reality:
- blood & energy going to digestion is going to take those resources away from attacking/defending.
In Tai Chi, it is usually recommended to do morning practice especially before eating, so the body is "light", and I've found it improves practice. (My own teacher would generally not eat a full meal throughout the entire day of teaching and practicing, and only do so at the end of the night.) But you need to have at least a little fat to burn if you're not going to eat a sufficient meal, and nutrition is important between training to allow the body to heal and strengthen.
However, we always used to hold that bagua, and walking the circle especially, is a great remedy for an overfull stomach, and my personal experience is that pans out.
In a real world self-defense situation, you don't have the luxury of knowing when an attack is coming, and you may have just eaten a large meal, so you must be capable of prevailing regardless of the specifics of your present condition.
(Many martial arts masters however, in my experience, tend to eat sparingly in general—I don't know if this is strategic or a function of expending energy more efficiently such that less food is required.)
It's only in competition fighting that one can determine when one eats in relation to combat.