There's a lot of threads on this from over the years which would be worth reading through as you'll find pretty much most of the main mods covered in a lot of detail.
Think most (but not all) would agree that an LSD makes a massive positive difference especially in the wet.
However, I never had any problems getting the car around quickly including on a track with a racing driver, so I figured the benefit for the times when it was wet or slippery weren't really worth the outlay.
I also read of them not being quite as smooth at slower speeds as the standard diff which put me off.
Similarly with the TCU update... makes slower speed changes a bit less smooth.
I didn't want to risk upsetting the car and I was probably only driving the car hard 10% of the time too.
I believe in learning how to handle a car i.e. with weight transfer and how to drive round corners quickly as well as a good set of premium tyres!
I also saw an employee of Quaiffe in Argos once and he was fing rude to them so that also annoyed me.
If you want a sub £300 LSD that actually works really well, look at Kleemann 60% locking LSD. I've had it in mine before changing the diff and it was fine. A forum member bought it off me and was very happy with its performance.
That's something else. The one I'm talking about looks like above and has p/n 051 113 01 37. It's two blocks and a set of spring. The springs push the blocks apart and lock the planetary gears. Very clever design actually and it works. Some machining to be done to the gears to fit the blocks but nothing major.