85 - 90% of hunter movement is the horse's conformation.
Another 10 - 15% is training. A horse that's relaxed, forward, and pushing from the hind will be a better mover than one that's pulling itself along on the forehand or one that's disconnected back to front.
I think with Sandie's dressage training, she probably meets those criteria, and is moving pretty close to her potential.
As for aluminum shoes, they help tremendously if some of the knee action is caused by being sore footed or bruised, having tender soles or wearing heavy steel shoes. If the horse moves with a little knee action because they are conformed to move that way, well, changing shoes won't help.
However, it's not an expensive or invasive thing to try, so why not try it?