Audi's launch of the S4 and BMW's launch of the 7 Series hybrid, combined with the diesel and gas DI 3 series offerings provide an interesting look into efficiency gains.

Numerous sources suggest that Direct Injection offers a 15% gain. But when combined with displacement changes and forced induction, this may be more like 20-25% when the base engine was especially thirsty.

Diesel seems to offer a 20-30% advantage (depends on driving conditions) over DI.

Hybrid adds another 15% (much of which may come from stop/start) according to BMW.

This suggests that a 50% gain is possible if we use diesel hybrids. A 30-40% gain seems possible with DI hybrids. If the typical performance car (M3, C6) gets about 20 mpg, then high twenties to low thirties should be possible with about the same performance.