Impacting performance isn't quite the way I would word it. There is going to be EMI and other noises inside of a system, dependent on the components inside and their proximity to the card, etc. That said, Asus is not stupid. They have thought of this and have factored it into their design. If we had multiple systems with various components and configurations, we might be able to measure it, but I wouldn't worry about the impact being audible. I have never been able to hear buzzing or anything audible across both of my builds that featured the card and with many different components and configurations.
Also, I have almost exclusively used the headphone output (except for a brief spell where I had my DIY Butte amp hooked up) and have no complaints whatsoever. However, I've always had 250 - 600 ohm headphones on my computer, so the output impedance hasn't affected anything. I would be interested if it actually measured at ~10 ohms once Amir gets his hands on it.