Same with the BMW V12's. They were meant to power big, heavy luxobarges comfortably up to 150 mph without straining, but they weren't intended for high HP applications. Those (like me) who have tried to turn them into cheap Ferrari wannabe V12's have wound up spending more than the real thing would have cost.
The boost approach isn't a terrible idea because the only way to get significant HP out of these engines with their poor head design is to "force" more air into them. I don't know about the Toyota heads, but the BMW heads have tiny ports.....
I think you're taking the right approach, however, if all you want is a DD with good torque at the low end. That's exactly what they were designed to produce.