Based on a lot of testing, an exhaust by itself will not provide much of a gain. For example, on a Camaro Z28 we did recently, a complete exhaust with headers only gained 1 (yes you read that right) one HP. However, once we installed the CAI it picked up 32 HP. So it always boils down to the combination of parts. Once you have the exhaust done, have it tuned with an SCT X-Calibrater 2 on your nearest Dyno to optimize the tune for those parts. The total cost should not exceed $1000.00 for a good Stainless Steel Magnaflow system and the X-Cal 2.
We have done 100's of Dyno tunes using the X-Cal 2 and it has always proven itself in. Go check out the"image section" on our website at www.fastspecialties.com
and click on customer cars. We have a write up on what was done and what it gained. Also read the "staff blogs". Lots of good info on proven power upgrades.