Anyway, I feel that this topic is very off track. Title says lens sharpness but more than half of the posts talk about focusing errors? Even a properly focused picture may be soft due to lens quality, although yes, an off focus pic is definitely soft.
Anyway, after being a pixel peeper for so long, I still must say lens sharpness is subjective, and unless you're a mega size printer, or always cropping due to lack of range on your lens, you probably don't really need perfect sharpness.
Also depends if you're talking about center or corner sharpness... whats the definition of sharp in this conversation here?
To TS, which range do you need more really? 28-75 naturally suffers a little at the corners wide open, although multiple reviews claim that the center is comparable to 24-70L. 17-50 known to be sharp wide open, but if you ask me, this applies to 17-35 better, the 50mm F2.8 part suffers a little... but really, it depends on how you use it.
If you're comparing between these 2, just choose the range you need. As for how to test for sharpness... you can just shoot flat against a newspaper, the 45 degrees thing is just for focusing. When shooting flat, pixel peep the center/ corners, every part... Don't have unrealistic expectations about the corners though.
Just my 2 cents, everyone has their own way of doing it. You can also design your own test chart and bring it there.
(psst, btw if you know the price for 28-75, please pm me thanks ^^)