There's a Japanese blogger by the name of Maro that is a very avid user of Sigma cameras, but he also owns a D800. While he doesn't do any scientific testing, he does take the same picture with each of his cams at many different settings. I found one link that compares the same scene with the D800, SD1, DP2 Merrill at full resolution. He actually has many other comparisons such as this one, but you shouldn't expect things to be all equal across the board (prime lens vs prime lens, etc). Even though, I think this is a very good reference and service that he does for his readers!
Feel free to take a gander here: http://maros-notebook.txt-nifty.com/notebook/2012/07/nikon-d800-vs-1.html
Naturally, it's all in Japanese, but the captions under the pics should be self-explanatory. Personally speaking, from looking at the many samples of the D800 vs Foveon samples he has on his site, I think that the D800 can resolve very similar amounts of detail. What I *do* notice, however is that the Merrills are able to capture fine texture better than the D800. Things like the gravel on the road, or the texture of roof tiles or bricks. I'd imagine that this is perhaps a result of Bayer interpolation on areas of similar color but with texture. The Foveon technology just preserves these texture details more than a Bayer would, I think.
Btw, full discretion, Im a Canon 5d3 and DP2m user.