Yes you're right that 5D pixels are considerably sharper than 7D, so one could probably upscale more there. I have personally more experience with 7D prints than with 5D prints so far.
Just for fun, here's an example of a picture from the 7D, shot with TS-E 24mm II at f/8. There's some tilt, maybe a little shift too, don't remember for sure. According to my experience this is as sharp as it gets on the 7D with this lens. 100% crop without sharpening to the left, 100% crop with mild deconvolution sharpening, and then the full image to see from where the crop is taken. For a high ppi print (300-400) I would probably sharpen a bit more, with some halo even, and for a low ppi print (less than 200) I would probably not sharpen at all to avoid pixelation around high contrast edges, but it depends on print technology and what the printer driver does too.
The 7D has 4.16 um pixels, I would not really mind if fullframe is eventually pushed this far (46 megapixels).