There are a few things with some good results.
CPR6 Performance Manifold - Gains of up to 25hp, expensive though but the price is coming down over time.
Mace Manifold Insulators - Engine runs cooler and does perform better as well, check out Mace in the sponsor area.
Pacemaker Headers - Unfortunately I've searced and found no one has done comparison dyno runs but people do say that the car runs and sounds better (Unless someone can correct me)
Stainless Steel Headers on ebay - has dyno results and increase of 4.6kw
CAI - Airhammer seems to be the best out of the pods, might get 10hp without tune
OTR - a few places offer this as well, seems this gets very good results with an OTR after a tune. Results I've seen on another forum.
I'm sure people have tried most if not all of the above mods and have posted results, a search will help you out there.