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At the risk of sounding smug
, one albeit much more expensive but extremely effective way of reducing understeer and increasing traction/grip is to install a Quaiffe ATB diff. It truly does transform a Mk5 GTI's handling at all speeds in both wet and dry conditions.

Sorry, but completely disagree. Grip and traction is only determined, in the final instances, by the abilities of the tyre. You can have the best possible diff in the world, but if you have shyte tyres, you will still be heading for the ditch. And do tell us how a diff will help with braking grip, aquaplaning resistance, ride comfort, tyre noise - not to mention everything associated with the undriven rear tyres? 
Yes, the Quaife ATB diff does work, but you are "bigging it up" way beyond its actual capabilities.
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I completely agree with you!!
All I'm bigging up is that, imo, if you take exactly the same environment and conditions of road surface and tyres, and were able to test the car with and then without the ATB diff installed, then when ATB equipped, the car (Mk5 GTI) would handle it with far less understeer and better grip/traction.
Of course this doesn't apply to aquaplaning, braking grip, etc - Surely you didn't think I was claiming so.