The fuel rating in EU is figured in a different manner from in the US.
Most countries use the RON rating, whereas in the US we us the (R+M)2 rating system.
US Octane numbers run 4 or 5 digits lower than EU for the same fuel, so 98 octane EU is the same is 93/94 octane in the US.
Therefore there is probably no difference in the engines. Run premium unleaded and you will be just fine.
I HAVE heard though that if you run premium instead of midgrade the FSI engine will run in FSI "mode" longer, giving you better mileage. I don't know how much truth there is to that
There is a good article on octane ratings here:
http://en.wikipedia.org/wiki/Octane_rating