From my experience and many others I know that print for a living or create images daily, we all agree that anything over 360ppi is wasted information. (both on Epson and HP printers) The images run the gamut from med. format digital backs and digital slrs to scanned 6x7/ 8x10" transparencies. The print sizes they have tested also run the gamut from 8x10" to 60"x 90". If anything, most of us tend to use 360 ppi for 20x30 and smaller, while using 270-300ppi for larger sizes. This makes sense since viewing distance changes with print size. I've heard of extreme cases where 600ppi for a scanned 8x10 tranny printed smaller than 16x20 actually helps, but according to them it's very slight. One thing to note, all these folks are printing color images on glossy or satin papers, B&W may require different resolutions. Also, many of the people using Epsons have used rip devices or software.
This info by no means is large enough of a sample to be scientifically significant but I have yet to see an image that benefitted from 400ppi or more of info. There was some anecdotal info for a few years on Epson printers, that one should use a factor of 9 for resolution (270, 360ppi, etc.) I've tried these and other resolutions that didn't meet this criteria and haven't seen a difference. Others say they thought there was a difference, but again, ever so slight. On my z3100 I tried prints at 270, 300 and 360 (600ppi native printer) to see if 300ppi would be better since it is exactly 1/2 the native ppi. Again, it didn't seem to matter.
I've also read over the years and don't know if it still holds true but native digital camera files (vesus scans) can use lower resolutions because the files aren't second generation like film scans. In other words, you don't need 300ppi to get the same quality as a 300ppi film scanned image. I don't have any personal experience that could validate this.