A 15% improvement is huge. It would save you $2600 over the life of an Aura (which is worth $1800 up front, at 5% interest, so the beancounters would correctly vote against it), or it would turn your 40mpg into 47mpg.
This is the kind of thing that could be used in addition to a hybrid electric system, like Honda's lean burn hybrids and Toyota's Atkinson cycle ones. In fact, here's an article from 2005 about Honda's HCCI efforts, which I'm guessing didn't lead anywhere:
Honda's Experimental Hybrid May Help in Race With Toyota - WSJ.com
"The aim is to tap, on average, as much as 40% to 50% of the thermal power generated by a gasoline engine per unit of fuel"
Regarding customer acceptance: Just have the car's speakers play a diesely sound when the car's not in HCCI mode. Heck, you could even play it during idle stop, if that's what the consumers want to hear.