No need to put extra weight in until you go for lipos.
Set the diff as per instructions. Loosening or tightening the diff isn't really a tuning option. You set it so it doesn't slip and all adjustments are made through the slipper clutch.
Things to buy first would be a spring kit, a set of pistons and some shock oil in varying weights along with a set of the tyres being used by everyone else at the club.
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