Spitfish works quite well as long as the track is hot enough to fall within its detection range.
You can also use the Sonitus multiband compressor in a pinch.
But far and away the best method, especially if you're dealing with a particularly nasty sibilant track, is the old-fashioned way: clone the track, brutally EQ the clone so it's all in the 6-8KHz range and compress the hell out of it. Then use that mutilated copy to drive the sidechain input of a compressor on the vocal bus. You can use the Sonitus compressor for this.
All else is in doubt, so this is the truth I cling to.
My Stuff