You'll shorten engine life with velocity stacks, but you won't make the bike faster. Vacuum carburetors (that's all OE carbs on all XS650 models) use the difference between low pressure above the slide diaphragm and atmospheric pressure below it to lift the slide. Air currents moving across the back of the intake bell can lower air pressure at the port that vents the lower diaphragm chamber. That's why all manufacturers use a still air box with vacuum carburetors, even on balls-to-the-wall performance machines. You won't gain power by hanging velocity stacks on OE XS650 carburetors, you'll lose it.