Slight bias but the EJ20 in Imprezas is highly tunable. I've heard reports of close to 400bhp on stock internals. Though I would take that with a pinch of salt I have been in 350bhp Impreza with stock internals (bigger turbo, exhaust, remap).
I know nothing about them, but when I started reading about tuning cars the Escort RS Turbo kept popping up. I understand they were/are highly respected for their tunability.