You can keep the first part of the primary pipes at the correct diameter (same as the exhaust ports) and increase them downstream. We call them "step-design" headers.
Right now we make them for the Ferrari 430, Porsche GT3 3.8, Lotus Elise/Exige Supercharged, Nissan 370Z and M3 E46. We also tested one on the RS4 V8 but it didn't go into production because the tests were inconclusive, the engine wasn't reliable (the intake manifold got dirty with oil after some time and the performance dropped)
It's been developed for engines with a wide rev range, to not sacrifice either lowend torque or highend power.
We could develop one for the D-4S engine, with longer primary pipes a bigger diameters, and a single, 200 cpsi cat. It won't be cheap but it would probably be the best chance to get over 220 hp without cams or anything else.
Based on this picture:
I'd say the exhaust use a 4-2-1 manifold.
Given the revvy nature of the engine, a 4 into 1 step-design header with longer primaries and a sports cat placed where the secondary stock cat is would be my best bet.
A sleeve joint to allow the variation of the lenght of the primaries caused by the high temperature may be needed. Not as much as in the Impreza turbo header but still.
The stock header is not stainless steel but mild steel so it has a lower lengh elongation constant.