I recently added a dual take-off GT exhaust to mine, and notice a definite HP jump, especially when the temperature is over 40 degrees or so. I had a lag off the line into first gear (manual), and since my exhaust work that lag is gone, as well as the the fact that the car feels even peppier than it did before. Cost me $140.00 + S/H for the GT exhaust off eBay, $40. for the hangers, and then the cost of the labor to have it all installed. I haven't actually calculated it yet, but I think I've gained a little in gas mileage, too.
-Lynn.