I've seen very good large prints from FF + 17-40L. Granted, it was stopped down. And granted, I believe crop + Tokina 11-16 would actually yield more detail and sharpness at the edges and corners. (Talking about 21 MP vs. 18 MP sensors.) But it's not night and day, and the 17-40L is neither a bad lens nor a lens incapable of 24" prints.
Whilst I agree, please read back to the start of the answer for full context.
I had a 17-40 on APS-C for around 3 years and loved it's build, and it's af speed. We are dealing with a particualrly demanding OP here, wants to view his prints with his nose touching the print etc..
I just don't think the 17-40 is going to live up to his expectations, in that context.
Would I have a 17-40 again? probably not, I need the f2.8 for video now, and as good as it was on APS-C the 18-55 IS was better.