One thing to keep in mind is that 99.8% of the time, it's not the lens resulting in a loss of sharpness. It's less than optimum conditions, settings, photographer skill, lighting, etc, that result in a less than tack sharp picture.
If you aren't a true professional who actually NEEDS the true best of the best, I think any of the "higher end, but off brand" lenses (Sigma, Tamron f/2.8 lenses) will be wonderfully sufficient for most any photographer.
I shoot a Canon Rebel T1i, still just have the 18-55 variable aperture kit lens. By taking my time and setting things up right, I've pulled some VERY sharp shots out of it.
Just my two cents, hope it helps a bit!