The most sensible way to discuss is to compare say Canon lenses where the same optical model is available (at the same time) with or without IS.
Nice, in theory. But in practice, the addition of IS changes the optical formula and the other elements need to be changed in concert, so there's no real direct comparison.
But it's likely easiest to make a general comparison with the 70-200mm zooms. In descending order of from sharpest to least sharp, they are:
- 70-200mm f/2.8L IS II
- 70-200mm f/4L IS
- 70-200mm f/2.8L
- 70-200mm f/2.8L IS
- 70-200mm f/4L
The fact that two IS lenses are at the top argues that IS doens't need to result in a loss of IQ...but those two lenses are also the newset in the bunch, and Mt. Spokane's point about newer lenses being sharper is true.
What, no love for the 200/2IS? The sharpest of them all.
The 200/2L IS is quite sharp, but the 300/2.8L IS II is sharper
away from the center, as is the 500 II. But probably not a significant difference overall - they're all damn sharp!