most folks put the a6 runners (lower intake with iab plates), a6 cam, a6 ecu, h22 plenum, h22 tb, max bore iab spacer (cuts heat transfer down and adds hp, keep the iab imo), bisimoto heat shielding intake gasket (cuts heat transfer down, adds power), 2.5" cat back, TRUE high flow cat with 2.5" or 3" inlet outlet (never use a gutted cat or straight pipe), a4 tubular header or aftermarket header with 2-2.5" collector, bisimoto's $1000 header adds 25hp alone...but that's pricey...don't spend a lot of money on any header but bisi's...and if you get an aftermarket one...get one with the larger diameter you can...they are usually sold in 1.75" and 2 or 2.5"...get the latter IF you can find 'em

I saw an episode of a tuner show where they went from 157hp to 200hp (car was rated at 210hp brand new) by doing the following:
Brand new injectors (replaced 10year old ones), brand new NGK plugs and wires (properly gapped and indexed...might help might not to index), castrol or better synthetic motor oil (castrol, royal purple or amsoil, or torco...from least to best)...importance being synthetic (don't use mobile 1...has a bad rap), brand new air filter (if you want you can cut out the intake resonator in the fender and open up the two holes on the filter box to be one large opening...use a paper filter...go to bobistheoilguy and look at his air filter test...you just need a NEW CLEAN one), top engine cleaning, tb cleaning, intake cleaning.