A lot depends on what sort of water you are fishing through. Bubbly, fast water where the trout has to make a decision there and then or the fly has gone generally means a positive take even on thick, floating leader.
Flat, "frog water" as the Americans call it is a different story. A trout has an age to make up its mind and may be coming from a few feet down so can see the leader clearly as it comes up through the water.
I must admit, I have been having a nightmare trying to sink my leader recently. No matter what I use, it floats, even after degreasing and even rubbing dirt along it to give it a little "weight" to break the surface tension.
When floater fishing for Carp, a sunken leader is a must, but I guess 8-10lb line sinks eaaier than 3.