Well, while we will have the breadth of knowledge, and skill with the spells, we won't have the experience.
kind of like the difference between using a skill in training, and in combat. Even with Gamer's Mind taking the pressure aspect off, in combat there are often so many distractions, and things you need to do, dodging while casting and defending, helping allies, multiple attackers, etc.
Dueling with Moody is helping us with that, but still, will need some actual experience in combat.
While I'm thinking of it, could we arrange duels with Flitwick? Possibly dueling against him 'and' Moody? Short of dueling against Dumbledore, that's probably going to be the best we can do for combat experience.
Also, can we learn the Turning class ability?