You can squeeze a lot of power out of turbo engines but it sacrifices reliability. Just look at the 1000hp/litre F1 engines of the turbo era or 1000hp/litre top fuel dragster motors. That makes 300hp/litre look easy!
Actually a V-8 rx-7 swaps are pretty common. It offers a huge improvement in low end torque, something the high-revving 13b lacks. I'm sure Jim Guthrie did it so he could break the back end loose at a lower rpm and maintain a more manageable drift.
You can squeeze a lot of power out of turbo engines but it sacrifices reliability. Just look at the 1000hp/litre F1 engines of the turbo era or 1000hp/litre top fuel dragster motors. That makes 300hp/litre look easy!