Well, there's a reason why it doesn't look like her. She is moving in action, and its a screen "grab", so it turns out all blurry. Myuu, or however her name is really spelt, is also very flexible. So, I think that one is actually of her.
But usually, it isn't hard to spot the stunt doubles, just about most scenes that can be remotly dangerous to do, its a stunt double. Sad, I would want to do my own stunts
I would think that the producers need stunt doubles for the purpose of keeping the main characters in place. I mean, would you want the show postponed for a week because Mako-chan hurt her jaw off a fighting scene or another character tripping? That would be horrible!
Props to the cameramen, though. They keep the G-Rockets people unseen with the blurred part.
I'll try to find some pictures, but most likely I'll have to use handy dandy VirtualDub, which I'll do later.
I'm actually pretty sure that that isn't her in that picture, simply judging by the way she fights when it's her for sure. Her punches and kicks are certainly lacking, and actually, all of the sailors are lacking a certain grace when they fight. I doubt that anything that looks remotely cool is really them. Except for Jupiter's fight with Kunzite. Because that was awesome. Anyway, even though Miyuu may be able to lift her leg that high, there's no way that she could kick with that kind of power if she can't even make her normal kicks look realistic.