I posted in another thread about a link to a photography site that said between 150-300 point, there was no noticeable difference, using a loupe to view. I believe that to be true, afterall, I have a wedding portrait that size from a lowly Nikon d200.
I would find you the link, but I'm on a silly tablet and too hard to navigate as is.
I have a couple of large format printers, the largest 44". I have done a bunch of tests to see the impact dpi has on the quality of the print. 150 is lousy, 200 is better and 300 is the best. Printing at 150 dpi is like using a kit lens. Obviously, if the original image has very little detail like a portrait, then it is harder to see the impact. I used detailed landscape image for my test.
Printers print at a standard dpi. The Epson printer are set to 360 dpi, the HP use 300 dpi. If you send an image less than the native dpi, the printer driver will uprez to the dpi the printer needs.
The more native pixels you have, the better your print will be. With the 5d3, it's native size is 16x20. Anything larger and you need to manufacture pixels. The d800 starts with a native print size of around 21x27. Obviously it will produce a better looking print at anything bigger than 16x20.