I agree the conventional wisdon for Canons (at least to now) has been 300 is as big as there is a point in doing. I acknowledge that I have not uprezed a file to 600 to print, which for me I am only interested in doing for a 16x20 or larger size. That because the smaller sizes without a microscope just cant get better (maybe, but not with the images I have been processing so far anyway). I will try this to have my own experience. But then the uprezing causing problems itself. And are "you" (who believe in the 600 dpi file rez) sending 8bit or 16bit files over? My 16x20 print 8bit files at 300 are 83Mb. At 600 they will be 166Mb, at 16bit they will be 332Mb. From previous in depth discussions prior to this printer release by many quality freaks, 8bit/300 was generally indistinguishable to "more" in almost all cases. And the fact is uprezing introduces its own issues to balance out. That is my data base. Sometime in the near future will add my own experiment with this issue. In the meantime...
I have no problems with your approach and findings. In fact I have printed one of my favorite images at 300ppi and the print is fantastic. It's an image that doesn't need super detail beyond what I get @300ppi and I've not printed it very large either. I have also printed it at 600ppi, letting plug-in uprez, as well as 600ppi from Photoshop and they are all very good. The 300ppi version is best in this case IMO because I did a better job of sharpening. I'm more familiar with 300ppi images. Differences were slight and all acceptable. I was, and remain, the weak link as I'm learning this printer. This "test" was done on an 8x12" image so I would not expect huge differences anyway.
I'm preparing to print another image that I think will be the type that will show the differences between 300ppi and 600ppi if this difference is significant. I'll print it at 16x24 and it has many details, textures, shadow gradients, etc. I'll try it at both resolutions. The original file is very clean and prints extremely well on my other printers.
I've not printed enough different types of images on this printer yet to know when it may be best to go with 600ppi. So far, I've just been looking for the recommended "best" approach used by those whom I've come to respect. Like you, I need to try more images at different resolutions and see the differences for myself. In time this will come. I agree with your comment that uprezzing has it's issues too, and I'm anxious to know the facts.
Oh, and yes, I am using 16-bit files before the export. I work in RAW, then 16-bit at the native resolution of the camera. The files do get large when upsized and uprezzed.
Gary, you're way ahead of me on the use of this printer. I look forward to learning what works best for my image files and best suits my eye. I appreciate that many continue to contribute their observations here.