You can fit a ESL daughterboard, as per Type-R-Rus and then have it mapped.
This will cost circa £550-£600 (£295+VAT for ESL board, plus mapping).
The 97/98 tend to get a bit spikey on the boost as soon as you change the downpipe or fit an induction kit.
You can control this boost spike with a brass pill in the boost control pipework, but then you don't get the benefits of the modifications.
Cheapest way forward is 2nd hand Apexi PFC or ESL and map it. Both would work out similar money. You should see close to 300BHP if the JDM WRX runs 380cc injectors as standard or 300-320BHP if running 440's.
I'm not sure what injectors/turbo the JDM WRX run (my guess is 380's and a TD04) as not many were imported. 99% of imports of 97/98 were STi with 440's and VF22's.
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