Mine has been set up for comfort and fast road use rather than a track car, hence staying with the stock rear diff.
The suspension set-up on stock Series 5's is dramatically different to the earlier versions. It didn't suit me at all, possibly spoiled by the way the GTN rides and so looked at ways to make a difference.
Have you made any changes to the GTN Shooter?
Curious to know if that needed any fettling.
