im late to the discussion , but i belive another factor that needs to be considered is manifold piping diameter and overall volume. i compared an ERC manifold with larger diameter and longer length piping against a factory S5 manifold, same turbo (with factory housing bored out for a p-trim). The ERC manfold with .84 housing spooled 1000 rpms later and was laggier all around. I did not compare peak power though
The S5 manifold's runners are about 1.3" equivalent ID at its narrowist area (oval shape). The ERC manifold runners were 1.75" ID, longer runs, and a lot more volume
The thing is, if you measure the exhaust port in the housing, it is only about 1.3" area, matching the area of the narrowest part of a factory S5 manifold. i have trouble understanding why you would go with larger ID manifold piping, if the port is the bottleneck (assuming stock ports). though most manifold builders use 1.6" ID or 1.9" ID instead
ive heard the reason it expands out at the diffuser exit is to keep egts down. i would like to try building a manifold with short 1.3" ID runners and dual wastegates to see if it could outflow the factory S5 and keep same spool characteristics