It's very hard to get gains out of non-turbo cars. Turbo cars can simply hold the wastegate for more pressure, which isn't always optimal, but as long as there's some leeway in the fuel injectors, it can often work just fine, especially with turbo-back exhaust, etc. With a NA car, you usually have to do things like cams, full exhaust headers, intake manifold, etc., these usually involve significant engine work and at that point, you might as well just go for a "built" racing block/crank/pistons at the same time. It's not impossible...but it's much harder.