I spent a year working for a rally team. It was not what I expected exactly, but was tons of fun. The cars on one hand take a ton of abuse, but on the other hand, are built for it.
Another year or two on the rally circuit remains on my to do list.
Excessive volumetric efficiency and optimized power to weight ratio.
Just about anything can be fixed.
All the technology that makes race cars fast, can also make every day cars hyper fuel efficient.