I think there are really 2 problems with this vocal. The first is that there seems to be a bump in the upper mids. So the vocal sounds muffled when the sibilance isn't jumping out. You just need to get the 'S's under control, and then take that lump out of the upper mids (800 to 1000hz I'm guessing?). Or if you cut the highs to try to get the sibilance under control, you might have gone too far. Also, double check your compression settings. If you're getting a ton of gain reduction and make up gain on your compressor, you'll make the Ss that much worse.
The sibilance could be a mic placement issue as well. You could try getting the singer to back off the mic a bit, or try a slightly downward angle. Also, a de-esser doesn't need to kill the air around a vocal if set correctly, since it only kicks in during the sibilent areas.