1) It won't be too expensive. You could set whatever regulations or limitations you wish on engine configuraton. Keep the regs exactly as they are today, but rather than a fuel flow limit set a power cap, even at 900bhp.
2) I agree, but that would depend on how open the regulations are.
3) Yes, the performance difference would be seen on Sunday. Assuming 200kg of fuel is required to run a race with 1500bhp and car 'A' is 10% more efficient than car 'B', then car 'A' will start the race 20kg lighter, which is about 0.8 seconds per lap faster. As fuel is burnt and weight decreases, the difference in lap time will also reduce.
Basically the only differentiator then is efficiency?
We wont see any laptime improvements in Q or sexy HP increase numbers I suppose.
I i doubt whether the manufactures consider this interesting enough to differentiate their product.