I used to own an iPF5000 printer, and now I have an iPF8100 printer. I discovered and helped diagnose a design fault with the paper feed rollers in the automatic roll unit of the iPF5000 printer with two Canon printer technicians, and we took apart the iPF5000, iPF5100 (identical to the iPF5000) and iPF6100 printer and studied the paper feed path and mechanism. They are based on the same design. The iPF6300 is identical to the iPF6100 too in terms of the feed path. I do not know the exact details about the 6400 or the 8400, but I am quite sure that they are similar to the models that they replace. If someone else knows otherwise, please correct me.
I have attached a diagram which shows the paper feed path of the 5X00/6X00 printers and the 8X00 series printers. I prefer the feed path of the 8X00 series printer because the paper's natural curl is not forced to bend into a straight path, as the Epson 9900s do. There was at least someone else who reported here having headstrikes with really curly papers, from paper bulging up after passing through the pinch rollers, because of the "memory curl" from the roll. I haven't faced any issues with curly papers on my iPF8100 printer, and I like this design for roll printing.
I really don't like how the iPF6X00 printers have to the bend the paper in the opposite direction of its natural curl. Fragile papers like Harman by Hahnemuhle Gloss Baryta can be easily marked. Other baryta papers seem to be tougher though, and I don't really see issues with the 6X00 printers generally speaking.
Unfortunately, as others have mentioned, all these large format printers are really designed to be used with roll papers. I have been testing some Canson papers recently, and I am very pleased by how quickly the papers flatten out after leaving the printer, for all matte, Platine and baryta types.
If you need to print on metals, and stiff board material, the Epsons are definitely ideal because of their straight feed path.