A decent cheap upgrade is a new servo, you can buy some pretty good ones from amazon or ebay for about $20 US they are just Chinese generic 35kg servo’s, but they are much faster and stronger than stock. Something like these will be fine...
Congrats, with some proper race tyres you'll improve the traction so much, get a hold of some buggy setup sheets online from Losi, Tekno, Mugen, ect and try to set you car up in a similar way. You'll find just lowering your car to a race height will give you a lot better control. Things like...