I would think that they would releive some since the spring is no longer pushing on the control arm to hold the car up, basically all the lower ball joint at this point is only keeping the wheel from flopping around.
in a coil over kit that is on the mustangs is a mcpherson strut set up the load is at the strut tower so ur caster plates will take the most abuse and the lower ball joint is a follower
in a coil over kit that is on the mustangs is a mcpherson strut set up the load is at the strut tower so ur caster plates will take the most abuse and the lower ball joint is a follower
still wont effect the ball joint... if anything.. the lighter spring is better for part longevity because the harder spring would exert more force to keep the tire in contact with the road and over the coarse of the lower Ball joint life span it would see harder impacts over bumps...where as the softer spring would absorb the jolt and transfer the force into the dampener(shock/strut).