A stock 998 produces about 30-40 horsepower. At this level the exhaust manifold, intake manifold, exhaust system, etc are not really a limiting factor. Much of the power increase people see from fitting extractors and a sports exhaust is due to the intake and exhaust manifold separation thus allowing cooler, more dense air to enter the engine. This is why, typically, most of the 'stage one' kits quote a figure of around 10% gain in bhp - it's about what you get from separating the standard intake and exhaust. And with these (now) old cars a proper tune-up can lead to similar 'gains' by effectively restoring the original power. As said, a good start with this is a decent electronic ignition module.
All that said, if you're planning more improvements in the future then fitting an exhaust, extractors, alloy intake manifold, good quality air filter, and any other bolt-on mod is effectively 'future-proofing' and you may see a slight improvement in top end performance...
But, as has been the case since its inception, the key to making power with the A series is the cam and the head. Both of which can also be fitted with the engine in the car as long as you're patient and prepared to cut out a slot in the radiator grille...
And the biggest thing - it's all good fun
