For the old WRX pre 1996 you could get whats called a VOS chip which slots into the spare EPROM slot inside the
ECU casing.
On standard injectors though I wouldnt take it past 16psi. Get some Sti 440cc injectors from the same era and you could take it up to 18/19psi. But by then I would recommend a new
ECU to keep it all in check. Apexi PFC has a PnP
ECU for WRXs of that era.
And yes, visit RSLC. Old WRXs shared alot of similar mechanical and electrical components. I am actually running a JDM 95 WRX STi Type RA
ECU in my Legacy GT (RS Liberty).
Get rid of the pre turbo resonator and replace it with a straight pipe. And get rid of the pre filter resonator also. The pre filter resonator ("snorkus") is highly restrictive.
Also check out a silicon turbo to intercooler pipe. Part No TCS22 from Samco Silicon Hoses. Old pipes by this time are hardened and can easily crack and leak.
Also the old plastic BOVs can crack and leak. My stock
BOV leaked pretty bad over 15psi.
Upgrade the fuel pump also and they are 10 years old now and wont keep up consistently with a 16+psi load.