g'day not sure how much you want to spend but a good start would be a k&n filter for just over $100 you get better throttle response fuel economy and smoother running if you want more it's time to get your cheque book out..
sjwau2xr6 - Tickford recommend that you don't change the heat range of the plugs for the I6 engine.
Only if you make SERIOUS mods to the engine should you even consider different plugs.
Hope this helps.
hmm power ups .. the best ive seen is a stainless steel air pipe running from the airbox to the throttle body , in our club 'Tickford Owners Club of W.A.' we have an ef xr6 that puts out 205 hp at the rear wheels with very few mods other than this style of pipe and exhaust mods (extracters etc) there is a picture of a similar styled pipe in the TOC of NSW tech pages under NOS ..