Old thread I know, but I am in a good position to answer. I have the 2014gt with 2.5. I have now had a loaner 2013 2.0 for over a week or 1/5 the time I have "owned" mine. Anyway, since my car was very fresh and new to me and this loaner is now fresh and new right after I basically test drove both for a little over a week. Here are my thoughts.
2.0 is not as doggish as I expected, and is peppy if you keep it above 3,000 rpms. That said the 2.5 feels more torqey and definitely has more of a punch than pep when needed.
I am getting about 2.5 more mpg with the 2.0, and I put some miles on the loaner in a real world work week. I believe it is truly 2 miles plus real world better mpg.
Passing power and dead stop power to start is where I think the 2.5 makes the car feel capable, as where the 2.0 feels ehh in those areas but other than that for normal driving they are fairly similar. It isn't like I was blown away by the power in the 2.5, it frankly feels like not enough, but at the same time it is welcome in situations over the 2.0, however I feel real world gas is more than they imply.