I added a Quaife LSD from Mountune back in July and it's the single most important mod for these cars, the only real way to use all the torque coming out of a corner, especially if you've added a tune (I'm on a stage 2 from
Dizzy Tuning). I went with Quaife because they've been doing automatic torque biasing differentials since the 1980's, they are geared (not clutched), and they are the Ford factory option in Europe.
I had a local Ford dealer do the install - I'm handy with a wrench but this was beyond me. All together I invested just under $3k in parts and labor but it has been money well spent.