I've had much better luck applying the fumed silica wet rather than dry. I even went so far as to buy a cheap blender to mix it. But otherwise, my experience with it is on a par with what Mr. Klain says - it produces a notable boost to Dmax and overall contrast increase. And applying it wet doesn't add THAT much time to the coating process. I find it dries to a coatable surface within minutes.
What's your mixing ratio, if you don't mind my asking? Thanks.