my 18G WRX puts down slightly north of 300whp now, compared to the stock 170whp. Unlike Ray, I did not got the route of the 2.5L hybrid build. As such, the lag is ferocious. I have to be very careful to remain in a useful gear, less get caught with my pants down. I can't expect any power until closer to 4K RPM.

While this is not AS applicable to the 2.5L Subarus, it is the case with Turbocharged cars in general. A larger turbo will increase power, but it will also shift your powerband to the right and weaken your low end. Technology like twin-scroll, AVCS, ELH, and Ball Bearing CHRA and billet wheel turbos will improve low end spool without impacting top end punch, but the general statement is still true. Just keep this in mind. FWIW, a car with turbo lag will feel faster to the butt dyno.