Approach angle will likely be the same, as the spring hanger will have to be in the same position the shackle was in to provide the same steering geometry.
You get a smoother onroad ride, but you lose traction while offroad. The tire will move slightly rearward as an obstacle is encountered and the spring compresses, unloading traction from the tire. In stock trim, the tire moves forward as the spring compresses and improves downforce and traction on the obstacle.
Give and take, you want a better road ride or more traction offroad?
The right way to do this is to sleeve the frame behind the axle to hang the shackle from, merely bolting a shackle hanger below the frame doesn't cut it. Mounting the shackle too low will greatly reduce your ground clearance, FUBAR's the steering geometry, and is downright FUGLY.