I have heard a lot worse sibilance on pro records but if you want to de-ess download Spitfish. http://www.digitalfishphones.com/main.php?item=2&subItem=5
It's one of the best freebie de-essers for vocals out there and it's free which makes it a great value product.... can't go wrong if used in moderation. Some of the other free fish plugins available via link are useful too. Remember you need some ssss for any song to sound natural. You can of course use a multiband Eq and search for the frequency of the offending esses by reducing the gain on that particular frequency you can also solve the problem. I remember reading an interesting article on sibilance in SoS mag and have managed to find a link.
Let me know how you get on.