There is bound to be a downside in addition to cost. In a rear wheel drive car without a
LSD one wheel loses traction and the rear starts to slide out relatively tamely because
the other wheel is still providing grip. With LSD traction, therefore grip, is increased so
you can zoom around corners faster, always nice in racing. The downside is eventually
the traction fails and now both wheels lose grip so the oversteer is a lot more dramatic.
Fine for racing drivers, maybe not for the general public. Hence warning notices in the
manual of my M3. I assume the same is going to be true for front wheel drive. Go into
a corner and break traction on one wheel and you get understeer at a relatively tame
rate as the other wheel is still helping out. Do this with LSD and both wheels will give
up together and you'll get big time understeer? Not that any of us would be tempted to
get back on the throttle too early leaving a corner
.