Well, Id check out the IACV and inspect the coils. Look for any hairline cracks in the casing. It may explain your flat spot at 4K rpm but I think the cold start issue is a sticky IACV. Pull it out and clean it.
Doing a Bosch HEC-716 Coil conversion is a huge upgrade over stock for early GC8s and RSs. Its much cheaper than getting OEM replacements and very simple to wire in.
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RS Liberty - Because WRX's are only good for parts...
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