i've got the TCR kit (very similar) and i'm looking at the Vspec hop up kit.
http://www.cmldistribution.co.uk/yok_tc.htm
its about 65 GBp, so it's quite a big kit.
otherwise, i got the silent rear belt and the soft front belt, and a tamiya suspension set, i found that the ones included were not of much use.
also, i have made a few weight reducing mods with a drill, knife and sandpaper