Depends on your goals...250hp (turbo) at the wheels is attainable with a good tune on a completely stock gsr block. If you want to beef it up, you can buy sleeves, pistons, rods, cams, valvetrain kit, P&P, there's so many options.
If you don't want to turbo and would rather stay n/a you could get intake, headers, exhaust, cams, valvetrain upgrade kit, head package, higher flowing intake manifold, TB, shit there's a lot of options depends on a specific power goal without a number it's hard to help.