Gem,

All that 'sink' stuff is basically a 'wetting' agent that allows the water to flow effortlessly around the surface. It basically defeats the 'surface tension' of the water.

That's great for getting a fly through the SURFACE of the water. Once there, however, how deep or fast a fly sinks is efected by all the same natural laws of physics and hydrodymanics that effect everything else.

Weight, drag (from both current and the line tied to the fly), 'shape', and material all effect both how deep and how fast something 'sinks'.

By the way, if you can find a pure liquid 'detergent', that will work as a 'wetting agent' and help get your flies under the water.

Also, it's often helpful to apply such things to the tippet as well, as that can have a large effect on how fast/far things wil sink.

Good Luck!

Buddy