As you suggested I printed at 720 PPI from LR with my cropped original 5DII file. Now I realize that this file is smaller than optimal for 720 printing but from my past experience there should be some visual appreciation. I once again want to stress that this file, as others, are tack sharp and in absolute in focus. This is extremely important for any upsampling. The result out of LR was very disappointing..... soft, lack of fine detail - in fact worse than the 360 print.
I took the same cropped image and upsampled in Fractals - same sharp settings as before - to 720 PPI. As you know this is over 21000 pixels in the long direction from a file that originally had only 5616 pixels.....that's a lot of upsampling! I sharpened that file in smart sharpen 300, 1.0. I viewed at 25% and could barely see a difference but there was some....... That printed image was almost night and day compared to the LR image with the PS image showing much finer detail and better sharpening. It appears evident to me that its the upsampling techniques and subsequent sharpening that are directly affecting the printed image. I have always been unhappy with the standard PS upsampling which is most likely used in LR. I know that On One has integrated their Fractal program for use with LR so that may offer some alternatives.....I don't know if that file has to be saved or if LR will store that step or if PS has to also be integrated into the mix. All I know for sure is what my eyes see...... Fractals is very necessary for my workflow. I really suggest that you try all of this for yourself.