As standard the design is pretty poor...lowered cars suffer from poor handling and increased tyre wear.
i find polybushes on the semi trailing arms give poor feel to the handling, the standard ford bushes work very well...but obviously on a car that's 15yrs old or so may be tired, so need replacing. They are available from Ford but are around £300 for the full set.
LSD's work poorly with age and need beefing up really..but in all honesty if you are sympathetic with them then they last forever, they may not work perfectly as they should but most drivers don't notice and as said, they literally go on forever if you are nice to them
Some people upgrade the diff mount to a Grp A type, this increases noise in the cabin though.
Also some people use solid mount's for the beam, again increasing noise/vibration, but improving handling.
Driveshafts are very strong as standard, so unless you go over 500bhp they will be fine.
i have heard of trailing arms bending, but it must be some seriously hard launching to bend them as most cars break traction before they bend anything.
there are upgrades for every part of the cossie rear axle, just depends what you want
