Re: AU diff gearing change
With a manual XR6 you should have a 3.45:1 diff and 225/50R16 tyres so at the moment fourth gear should be giving you about 33.1kmh/1000rpm. In top 46kmh/1000rpm. Its already fairly low and without serious mods to allow the engine to rev right to the 5900rpm cut out big gains won't be had going lower. Lots more wheelspin in first maybe and slower times or faster times taking off in second. A 3.73:1 in a standard AU Auto drops third from 36.8 kmh/1000 to 32.1kmh/1000rpm which is good it gets it to an XR6 spec. In an XR6 I don't think its really going to be worthwhile but it will drop fourth to 30.6kmh/1000rpm and top to 42.5kmh/1000rpm. Your speedo will read slow but a change to a black speedo drive gear will fix that Part no.XE17271F, $10 from Ford.
How much faster well at a guess you will drop the 1/4 times by 0.4-0.2 seconds not much but if thats your game!
Money wise wreckers prices are pretty negotiable and it can be $250-$500 odd for the centre. maybe the same agin to fit.