the HPI's require the following to go better/faster
-ajustable camber links for the steering/camber etc. (anywhere where there is a fixed link replace it with adjustable) just take the car the the shop and check the size, most brands are interchangable
-a better tuned pipe. any novarossi, mugen seiki pipe max, sirio pipe will do. skyline make cheap good pipes. always use a one peice pipe, don't use the pipes that join with a rubber bit and zip ties as they always come loose
-better tyres suited to the trakc you race at
-good fuel (this is key. get quality stuff, go to the premier track in your area and use the fuel the quickest guy uses)
-a glow plug suited to your weather conditions AND racing type (ie high speed, low speed etc)
-correct ride hight, droop etc for your track