Today my friend Rich and I wanted to get a good solid day of wargaming in. We played 2 games my grey knights against his tau. (yes tau, they are actually the only army with a winning record against me. when played properly they are absurd). The first was a game where he went first, but I still managed to win. The second was a game where I went first and managed to lose, oddly enough. Either way we had time to play another game but neither of us really wanted to play another grey knights vs tau match and I didn't bring my other armied with me. In the end he asked me to proxy up some dark eldar and play as them to give his tau a real test.
What I learned from dabbling into the darkness was all of the weaknesses and thought processes of the dark eldar player. Since I'm always on the receiving end of the army, I always see them from my perspective, which is this scary 12+ tank meeace with 25+ lances and 108 poison shots. Now that I've played as them I understand how hard it is to deal with an army that can shoot back, much like my grey knights can. How a couple bad rolls for damage, or some good rolls for cover could spell the end of the game. Despite its spammy nature, it's actually a much more luck dependent army that I originally thought.
The point is I know demons, tyranids, and now grey knights like the back of my hand. Against an opponent playing one of those armies I have a sever advantage due to my experience. So what I'm saying is maybe instead of play testing how your army works against certain armies, try plaaying those armies and see what they have to worry about and thought processes you have as your opponent.
I for one can tell you, against willing opponents I will be proxying some of the natiest lists I can think of to try and further my skills as a general