The 2.0L GDI Ti-VCT is tuned to adjust for whatever octane rating of fuel you've put in it. Since it is a high compression motor (12.0:1 compression ratio) it can take advantage of the increased energy density of high octane fuel and will produce more torque. If this were five years ago and the engine were fuel injected, it would require premium fuel to run properly. The advantage of direct injection, and more advanced control systems and sensors, is that you can run lower octane fuels without issue if you want to save some money.
I have noticed that I get a bit more low to mid-range grunt when driving on a tank of premium. This helps me get slightly better fuel economy around town, but when I'm doing days of highway driving, the octane rating seems to have less of an effect on fuel economy.
Be careful with randomly filling up any old car with premium, however. With some cars it will do more harm than good, or it will just do nothing. The Focus has a pretty snazzy engine, however, which is why you're seeing the improvements.
2012 Focus SE Hatch 5spd sport package, Tuxedo Black Metallic
Road Magnet springs and fuzzy dice on the mirror