I would give it more than "mediocre", a 15 percent increase along with more power is pretty amazing!! We all think are engine technology is mature with little left to improve on but....
remember, these are the new, lowered epa estimates, this car will be pretty much be at or above 40 mpg all the time. Our 41 mpg Corolla is now only rated at 37 under the new system.
Direct injection is allowing 12:1 compression, the technical reasons why this is possible are fascinating. Years ago I had cars that pinged endlessly with 9:1 compression, now we have gasoline engines running near-diesel like compression ratios, with the attendant efficiency.
Increasing compression give higher thermal efficiency, but it is not endless and further gains are much smaller. At one time we thought adding ethanol might let us get up to these ratios and overcome the mileage loss inherent in using ethanol. Now, it appears that we gave gotten there anyway, without the ethanol downsides