Hey, I know that stuff. I understand the relationship between aperture and resolution (both theoretically and from testing all of my lenses). I get very "sharp" images from my current gear that can be printed at quite large sizes, so I do understand how to make that happen.
It isn't as easy as "just shoot at f/8 to f/11." In fact, shooting at f/11 is _not_ the best choice for maximum resolution on a crop sensor body since diffraction blur can become an issue beyond about f/8. (f/11 or even f/16 can be fine choice on a FF body.)
From my own extensive experience with the 17-85mm lens I just can't imagine that it would produce the sharpness that I expect in a 20 x 30 inch print - you did mean inches not centimeters, right? I get that kind of sharpness from my current gear and careful shooting technique plus suitable post-processing, but even then it isn't a slam dunk - it requires care and attention to detail. I never came close to that with the EFS 17-85.
Here is an example of a photograph that I made with the EFS 17-85 lens on a crop body a few years ago. It looks pretty sharp in this small jpg and even in a letter size print. But it barely holds up as a 12 x 18 print, and then only with some extensive post-processing in PS - and the presumption that the viewer will not inspect it too closely. I would not sell a print that large from this image without making absolutely sure that the buyer understood this.
