I bought a very similar intake from Rallitek for my '99 RS a year ago:
Besides being the loudest thing I've ever heard under my hood, it also caused the engine to lean out a LOT. So much, in fact, that at 5500RPM the ECU would force a fuel cut as though I had hit redline. I ended up having to buy and install an S-AFC and run the following air/fuel correction curve:
(notice 21% extra fuel at redline). Then it made what seemed like some extra power, but I ended up selling it and installing an Injen. I'm more satisfied with its sound and it does not lean out the car as much, though its still around 11% extra fuel at redline.