First of all, in response to Pete - yes, I am quite biased. I am a quality nut. I don't do anything that's beyond the diminishing returns, however, I am careful of that. Honestly, I was sure that he was talking about quality and I think I got some threads mixed up. That said, I don't believe in the concept of "pro" shops that you suggest. These are usually just offset printers for whom the key idea is quantity. I am not interested, nor would I suggest that the OP start such a business. One needs a lot of volume to justify it.
Lenny, it's dawning on me that you are being just simply dismissive, obtuse, or unaware of the commercial archival large canvas printing world that uses, eg Fredrick canvas, and state of the art Epson 11880 printers, as does "180 ppi" Rick Forshino's Coastal Giclee catering to the large number of artists and photographers in the Monterey Peninsula area. And he's not alone. I doubt you do such work yourself, but I could be wrong. This forum has many who do the same quality work commercially or personally, and it would be nice to hear their perspective on the180 ppi vs 300 for large canvasses.
I just did a quick comparison of 300 ppi vs 180 ppi prints of Atkinson's very high quality printer test image - a pastiche of 15 or so small, highly detailed images and 5 color bars, patches, and a large grayscale gradient, 13x16" size, on my iPF5100 at maximum quality, using a finely textured satin paper. With bifocals at 18", I can see no difference except for slightly more vibrance in the 180 version, but it's fresh and the 300 is several days old. But using the superhuman vision in my greatly myopic left eye which focuses at 8" uncorrected, the finest detail is sharper, as expected, in the 300 ppi print.
Now this image at pixel level displays on my 94 ppi HD monitor 40.7x51.3 inches - perfect for the large canvas comparison. I took a 100% crop in an area of fine detail and subtle gradations - the yellow rose and the forest frens, upres'd first to 180 ppi, and with careful output sharpening, printed; then further upres'd and sharpened for the 300 ppi print. Not unsurprisingly, the results are similar to my close and "supernatural" vision in detail, color and gradation, but with a slight increase of vibrance in the 180 dpi print. Each of the 8x17" slices from the roll consumed the same 0.7 ml ink. I rest my case, and this using a much higher resolution medium than canvas.