The standard headers (manifolds) are responsible for producing that distinctive “Scooby rumble”. Mild porting of the internal passages can give some limited improvement to power and turbo spool up. For cars running over 400 bhp there are considerable gains to be had from fitting tubular headers and matching up pipe.
Most designs are unequal length and still retain some of the traditional “rumble” but blend into a super bike type wail at higher rpm.
Early designs of tubular headers have a two bolt flange to the up pipe which is prone to failed gaskets. The later designs now have a three bolt flange which has irradicated this problem. Heat proof lagging of tube headers is highly recommended as this improves power and reduces lag.
Some brands of headers have integral expansion bellows type joints (shown in pic) these have yet to prove themselves at high boost for a decent mileage. The top set in this pic (regularly advertised on ebay) blew apart in less than 2000 miles on our project WRX running under 1.5 bar boost ! The lower set (Megan) also failed within 3000 miles
This is the section of exhaust from turbo outlet to the start of the exhaust system. Fitting a decat downpipe is one of the first and most effective modifications that can be done. This reduces the back pressure on a turbo which allows quicker spool and increased power. The open neck design of downpipe is preferable to the twin dump style. This is due to the waste gate flapper partially obstructing the small dump pipe and reducing its effectiveness.
A fully decated 2 ½” bore system is adequate up to 400 bhp. After this a 3” system is recommended. Sports cat systems are available but ultimately still limit power compared to a fully decated setup