as much as 30 percent fuel efficiency over its Skyactiv-G engine
Exactly. When talking about ICE, one must be careful to distinguish peak efficiency, average efficiency, or efficiency at an arbitrary power band. Mazda picked an arbitrary power band that showcases the tech advance but that says very, very little about overall efficiency improvement.
About the best an ICE can ever hope to theoretically reach is current Toyota hybrid levels of about 55 mpg over a complete drive cycle. Getting there without a supplemental battery to flatten out the power curve and not be under-powered would be quite a feat* but that is the limit. At current fuel prices a car like that costs about 4 cents a mile. My LEAF, fueled by my home PV, costs 0.5 cents a mile.
Regarding the new Mazda engine, it sounds like they are trying to get rid of the air throttle and its attendant 'partial power' losses every petrol ICE so far developed suffers from. In effect, they are 'diesel-izing' the engine. That is fine, but the problem to date has been huge increases in Nox and particulates ... and then having to spend tech and money on emission control. The VW diesel story informs the Mazda story: look for the emissions profile and emissions controls it requires. If it is anything comparable to diesel, it will fail.
*It has been talked about. Presumably a turbo, along with variable displacement that does away with the throttle plate, could get there.