[aerohead]

I've not seen, nor heard of any Corvettes modified for efficiency in 45-years.
We have a local North Texas Corvette Club called the 'Glass Menagerie' by members.
Other than 'hop-ups', they maintain as much OEM configuration as possible.
Chevrolet Division does quite a bit to make them as efficient as possible, while maintaining 'trackability.'
The V-8 has been retained on account of its ability to function on 4-cylinders during low load conditions.
The hybrid model C-7 allows a few miles of battery-only operation.
The upcoming, all-electric model will be the most efficient.
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They already have 100% synthetic lubes, so no potential there.
There aren't any LRR tires available for them.
They run MOON covers on them at the dry lakes, but that requires steel wheels.
Which leaves only body aero. The roof already 'disappears', so that's already dead on arrival. The main body aero is destroyed by all the cooling ventilation and enormous rear haunches and transom.
It's a high-performance grand touring coupe, stable at very high velocity; designed for high lateral acceleration, and thermal management. There's no painless way to realize any meaningful gains in mpg as far as I can discern.
30-mpg at 65-mph is about as good as you're likely to experience. Tip into the go-pedal and it goes south fast from there.