This particular model is primarily for dirt bike use, wear it under a jersey or an enduro jacket, but it can be worn under leathers. My street leathers are cut slightly loose so that I can wear cold gears underneath so this jacket fits snugly underneath. There is a new model that only has the chest & back and cut like a vest. There is also a model that is supposedly cut for racing leathers with stretchy material instead of mesh.
you can wear that under your leather if you remove the armour from your jacket. ive done this w/ my a* jacket and my 661 brand sp2 armour, which is basically their version of that a* armour.
let me tell you though, its kind of a hassle. so i now only wear one or the other.
as far as wearing it by itself, i personally have fell @ 25-30 mph off a friends bike and slid about 10 ft. on the asphalt and it protected me well. my right shoulder and elbow armour plastics had gouges in them, but no road rash
i cant promise you it'll hold up @ higher speeds. but to answer your ??, you CAN wear it under leather comfortably.