Here are a couple of examples of what to really expect from upsampling.
I started with a crop of 450x450 pixels from a 1Ds Mark III camera file (Capture One conversion, no sharpening):
Then I've made 2 examples of 400% enlargements, in line with what the OP gave as a theoretical example (they should print as 5 inch square at 360 PPI). Here
is one with Photoshop Bicubic Smoother, a quality which according to Jeff "prints out like crap, lots of ringing and artifacts from the upsample". And here
is one example of a 400% upsampling with Photozoom Pro (S-Spline Max method).
Both are not sharpened yet, so if you want to print them, you can use your trusted method of sharpening. For viewing them on screen, they are best viewed at approx. 25% zoom for a size that resembles the printed output, but the quality should really be judged from a printed version, properly sharpened for the media used.
And then I've made 2 examples of an 800% enlargement, which would resemble the quality of a 3.17 x 2.11 metres output from a single 1Ds3 file at 360 PPI, or can be directly compared to the above prints when you use 720 PPI ("finest detail" option selected) as output resolution. Some authorities say there is no benefit to printing at 720 PPI when the original file has a native resolution of below 360 PPI. I say, try it yourself, and like me draw your own conclusions.Here
is the Bicubic Smoother version, and here
is the Photozoom Pro version. Both are only upsampled, not sharpened yet.
Of course there is no substitute for real pixels, but I'm looking forward to hearing what the findings are ..., does upsampling (sometimes as a last resort) produce crap or what?