by Adrian2627 » Thu Feb 23, 2017 3:55 pm
The EJ20 is arguably a better block than the EJ2.5 but doesn't seem as prone to the head gasket issue of the latter. There's a reason Subaru have stuck with them here in Japan until the FA/FB20s came along.
At 170k there's still life in it but a refresh would be a great idea if you plan on upping the ante. There are a few options depending on your budget including swapping in a used JDM STi long engine/turbo, refreshing your engine (rings, bearings, pistons, recon the heads etc), or starting with a fresh EJ207 STi block and add your heads, turbo etc.
Personally, I'd start with a new EJ207 (adding a Killer B oil pick-up and ARP head bolts), recon/upgrade my heads (new/bigger valves, work the seats, springs) and maybe upgrade the turbo to a TD04HLA19T or 20T. If budget allowed I'd add some STi cams to work better with the headwork/better flowing turbo. Your goals could be different but that'd get me a good increase in drivability without sacrificing reliability.