Soft L lenses dont exist. L lenses can back focus, or the resolving power may differ. L lenses are made to be sharp and they don't vary from sharpness usually. The only L lens that I had that was bad was the my first 135L, it front focused. When I did focus it correctly, it was dang sharp.
My experience with the 24-70L is that it didn't resolve as much as the primes I use. It was sharp, but didn't have all the detail there (atleast wide open like my primes do). I guess sharpness and resolving power run in tandem, but L lenses are generally sharp. Some are better at resolving than others.
