Doing a bit more research...the work that is being done on this at universities is still early and they are not adding H2 to the air stream but have to create a high temperature upstream plasma heat source and add the H2 into the liquid fuel stream to create a homogeneous fuel...when they do that, they get 20-30% better combustion efficiency.
Green Car Congress: Hydrogen-Enhanced Combustion Engine Could Improve Gasoline Fuel Economy by 20% to 30%
It also requires an increase of 2x the air, so I suspect that the engine components have been changed to handle some really high temps due to the extremely "lean" conditions.