Not much as I bought an S3 to stay standard, it came with KW Coilovers already on it then I had some money burning a hole in my pocket so fancied a new set of wheels. Aero blade all round, then the rest just spent on servicing/maintenance.
Mine is pretty low on Coilovers, just beware that the sump is pretty low so you need to take it easy on bumpy roads/speed bumps.
They were on it when I bought the car so can't really comment whether its worth changing from what you have other than you can obviously get a bit lower.
Yeah its defo the rear diff. The tyres are fine, they are being changed on the weekend so if it is them the problem should be eliminated if it is them. Its just like a whining noise as you ease off the gas.
I know the Haldex is a seperate system, so yeah i'll look into changing the diff oil...