If you're after performance then the exhaust side of things is the right place to start with. Indeed, power depends on the amount of mixture your engine is able to gulp in a given amount of time, and being basically an air pump, it can't gulp more than it can evacuate. So exhaust is usually the first thing to consider for other improvements to be effective. However, exhaust is quite a complicated science involving scavenging and back-pressure.
The first thing you need to do is to determine the use you want to do with your car, and within this your personal driving style needs also being taken into consideration. A track engine requires different work than a fast road engine, which will also be different than a touring engine.
We often talk about power, but the most important thing is torque. The way it is delivered determines the personality of your engine.
The design of a header will determine the rpm of your peak torque and the rpm range at which it is delivered. If all the pipes have the same length, then all cylinders will deliver their max torque at the same rpm, and the range will be narrow. the length and diameter of these pipes will determine what rpm that is. This is most suited for a track engine because you end up with nice torque figures, usually at rather high rpm, but given the narrow rpm range usable, you need a close ratio transmission (like the Getrag dog leg). This is not suited to road use, where you need a wider rpm range. This is why AMG was fitting headers with different pipe length for road use. The max torque figure was less, but it was delivered over a wider rpm range, resulting in a more useable engine.
The camshaft profile then needs to be assorted to your exhaust characteristics as it also contributes to determine peak power and peak torque rpms. Finally the intake manifold needs to be assorted as the diameter and the length of the runners also contribute to torque characteristics. Then the rest of the engine needs to be developed according to the characteristics of these parts (porting, size of the valves, compression ratio, lighter rods and forged pistons if the rest is tuned for high rpm etc.).
If you're just into a rebuild and light improvement, your options are more limited, as your intake manifold and valve size will remain the same. Compression ratio will also remain the same if you keep your pistons. You will be left with optimizing all this (balancing, porting and polishing) and the two parts you can play with are the camshaft and the exhaust. They will need to be assorted to each other, and their specs will be determined by what the rest of your engine is able to take. And you then need to see if these specs are different enough from your stock camshaft and exhaust to justify for the upgrade.
In the end, I guess, the best is to talk to people who went that way before you to save some of the try and error process. Peter Lesler has been racing a pagoda some time ago so he will be able to help you. Joe and Dan will also have some good clues. I would also rely on what Metric Mechanic says as they have a great amount of experience. But you need to have a good discussion with them on the use you want to do of your car and your personal taste, as options will be different.
