You will gain some rumble compared to the lack of rumble in the 06+ using the 98 manifold. To get decent rumble you will need a true header like a Borla or ebay Borla replica.
Mike is right. The NAs are equal length, but not as equal as the 06+. Our 07s are exact equal length, making them sound more like hondas, but Subaru gained 7hp doing that (ports remained continuous longer than the older manifold type). I think you need to disregard what the girlfriend says and get the eBay Borla replica headers, straight pipe all the way to the catback, maybe a glasspack/cherry bomb resonator...maybe. We used a cherrybomb on the RX-7 for a resonator, then another for the muffler. Worked good and lasted 5 Maine winters. Use the muffler you have on it now. I might be interested in your stock header with cat. I was thinking about cutting the cat off and doing a catless equal length header in my car, and saving my catted header for when I decide to sell and return to stock. I'm more concerned with those last couple HPs than sound. To me the stock header on the 06-07 NAs is the shizznit. It don't get any better until they jumped to the long stoke 2011 Foresters with the timing chain for the NA cars.
