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Further thoughts regarding the diff:
The Mk5 GTI has TC as part of ESP. The ESP can be switched off via a button in front of the gear stick. The ESP always defaults to being On whenever the ignition is switched on. On the road I always drive with ESP On - Imo it's not a matter of proving you're a man by switching it Off! But different cars behave differently.
Don't ask me about the exact structure regarding TC and ESP on the Mk5 - It gets complicated! But the TC icon in the speedo lights up every time the wheels are losing traction and signifies the ESP coming to the rescue.
One result of having a mechanical diff like the Quaife is that the TC sensor system is less frequently called upon to instruct the ESP (or however it works). In other words, the ESP follows the Quaife's physical responses rather than rely on electronic sensors which can either go wrong or get dirty like my Wheel Speed Sensors did and which probably then fooled my ESP into action which caused wheel snatching in compensation - Not confidence inspiring!!
Someone who knows more than I do may post that I'm talking bollox but I offer this as being to the best of my knowledge.
Having spent as much as £1,500 on any mod, the placebo effect naturally comes into play - You say to yourself (or your SWMBO) "Of course it works well - I spent all that money on it and of course I make the best decisions". However, the Quaife is something which has such a physical result that it's not simply your butt-dyno. Whether spending the money is worth it to you or not can only be your decision but I have no regrets and everybody who has either done the same or owned a FWD car with LSD already installed, has nothing but good to say about it - It's an often overlooked mod.
If your ownership of a Mk5 GTI doesn't occasionally involve spirited driving (as appropriate) then it's not really worth it. Installation involves open heart surgery (by someone who really knows what they are doing on DSG) and you will not see any money back - You ain't gonna be removing it come sale time! But I love driving and am enjoying the benefits and have no plans to sell my GTI - It's too good an allrounder - Fun and very practical.
If you love track days (I haven't driven any) I'd say that the Quaife and a big brake kit like AP's are a must!! £3-grand for those two mods though.