The strength of the upper arm shouldn't be a huge concern IMO, since it doesn't bear any of the load of the car. All it does is hold the top of the spindle up and in the correct orientation. That said, if it broke things would get ugly..
Uh, it doesn't bear the weight of the car in the vertical plane, but any steering, cornering, or braking loads from the tire that don't go straight up and down are split between the upper and lower control arms.