In the cruise, the traditional wake turb is neglible from the fuse etc. it is mainly wing tip stuff. Winglets reduce it a little because they blurr the mixing boundry between hi press and low press air around the wing, but they do not do enough to stop this.
At standard atmospheric conditions, wing tip vortices desend at about 500 ft/ min. If the aircraft is 1000 above u then you have about 2 mins. At .8 that equates to about 15 mls. In RVSM airspace, you can hear a 747 go over the top of you, 1000 ft above so u sure will feel it.
The 757 is a particularly bad one, so it has its own wake classification, not based on its wieght.
Just imagine going under a 380: (:-(((