Yes, at a podcast listener's request, I tried it. At ISO 200, images from the two cameras look virtually identical. The only difference is that if you look in the shadows, the D300 images will have a tiny bit of noise that's not present in the D3 images. It's not something many would care about, including me. Where the D3 images are better is in situations where you want to bring out shadow detail. You can lighten the shadows in Photoshop without getting ugly noise.
Keep in mind that one of the advantages of the D3/D700 sensor is that you no longer are forced to shoot landscapes at ISO 200. Many of us who enjoy landscape and nature photography have always shot at the lowest ISO to get the best quality, but it's remarkable what you can get at much higher ISO's. That added flexibility takes some getting used to, but the increased options are nice.