Equal length vs. 
Unequal length headers 

While this information is not specific to Subarus, we will explain the difference between equal length and unequal length exhaust manifolds using the Subaru example.

When modifying WRX/STis, it is important to plan for the type of the aftermarket Subaru exhaust manifold (header) your will be getting. Unequal length header will allow you to keep the Subaru Boxer rumble sound. Equal length header will allow you to maintain equal Exhaust Gas Temperatures among all cylinders and turbo inlet temperatures.

Since the header runner for cylinder 4 is the longest on unequal length headers (on WRX/STI), you have the highest EGT in that runner and on cylinder 4. This is because hot exhaust gases travel longer in the longer runner. Cylinder 2 runner is the second longest. Since you are continuously subjecting cylinders 2 and 4 to higher heat than other cylinders.

Shorter runners of cylinders 1and 3 provide for lower EGT on those cylinders. However, since the temperature drop is less than in cylinder 2 and 4 runners, higher-temperature exhaust gases get transferred to the turbo, subjecting the turbo to additional heat, which could be the reason for unequal length headers having peak power gains in midrange and feel more powerful than Equal Length headers.

Equal length manifold (header) smoothes exhaust pulses of the Boxer engine, thus changing the exhaust note to smooth vs. burpy. Some refer to it as a "ricer sound". Equal-length header is optimal for maintaining equal EGT between all cylinders and thus delivering a constant, lower-temperature exhaust gas to the turbo for their linear power gain.