With stock WRX heads and cams, yes. The 18G's I've driven on ported WRX heads and non stock cams was freaking awesome to 7000. Same on the EJ257 STi/WRX/Forester.
so you guys would put a 20g on a 2.5 block instead of the 18g? i was thinking of the 18g when i do my swap cause i don't want a ton of lag like i thought the 20g would have.
Stock turbo makes 18 psi at 2950rpm, 20g is 18psi at 3100... the 20G is the smallest turbo I suggest for the 2.5 liters... Siegel