Cheapest (and most important) is the tune. If you're only after a one off improvement then an HP tune would probably be the cheapest (no other hardware required, just an ecu flash by the tuner). If you want to run a couple of different tunes or have the ability to switch back and forth to stock then an Xcal3 is needed (and preferred by many) Should get you around 250+?? rwkw, but drive so much better (and even be more economical) across the whole rev range. Any work you do after this will require a re-tune so you're better doing as much as you can all at once. From there there is a few other mods that can take you up to 280, 300 and 320+rwkw (about where the standard turbo runs out of puff). You asked for cheapest - tune only is the only thing that satisfies that requirement. Give a budget and you'll get some answers that fit within it. More important is to work out where you want to end up and work with your tuner to find the best way to get there. ZF tune will be an extra $500 or so, but also requires an Xcal3 which is another $800 or so. (look up the Xcal3 to see the benefits they have over an HP tune). Probably not something you need unless you're unhappy with something the ZF does or want to improve it. Heaps of info here on what people have achieved with different levels of work and budgets. Keep in mind that any changes you make after the first tune will require a retune - be patient, do your research, find a tuner you trust and work with them to get the best result and insure you only buy things once.