First, the power. According to Mazda, the 2.0-liter Skyactiv-X makes 178 horsepower and 165 pound-feet of torque. By way of comparison, the 2.5-liter Skyactiv-G engine currently used in the Mazda3 here in the States makes 186 hp and 186 lb-ft of torque. Sure, it's a little less powerful, but note that the displacements don't match and remember that the real reason the Skyactiv-X exists is to reduce fuel consumption.
And that brings us to the Skyactiv-X's combined rating of around 53.5 miles per gallon (converted from liters per 100 kilometers). That's around 8 mpg better than the 2.0-liter engine that comes standard in Europe.
Before we go any further, it's important to note that these numbers represent official figures for the European NEDC combined cycle, which means they are not directly comparable to the ratings we'll get from the United States EPA. Our official ratings will almost certainly be lower. With that disclaimer out of the way, the 2.5-liter Mazda3 scores a combined 30 mpg here in the States.
The obvious unanswered question is when the Skyactiv-X might actually come to America. And the unfortunate answer is that we don't know. Mazda has yet to make an official decision on timing for the U.S market's launch of the engine, but as soon as we know, so will you.