Why do they need more power?
Is a requirement for all american cars that they must double as race cars during the weekends?
There are quite a lot of factors contributing, but I've been merciful and cut the size of this post down quite a bit. It seems like I'm not really answering it--and I'm not--but maybe for now, it's best to leave performance as a 'given.'
Fiat's not going to redesign their engines for us, and there's no need for them to, but I would expect nothing less than their 2-3 highest specced units for each platform, and for the diesels to mostly stay in europe for now. Without getting into American culture at all, every car (except the Fortwo) that the new FIATs will be going up against has at least 110hp. Performance is a really easy metric for people to wrap their heads around and instantly rule a car a total waste of money. It's not a wild bet that FIAT is going to be bringing mainly their hotter engines here for now.
I'm not saying this is going to remain a fact until the end of time, but as of right now, 110hp is about the minimum a carmaker can get away with. Yes, historically Americans have done with much less, but engine technology has progressed, and the weight of compact cars has increased. These new cars need a bit more engine to haul their weight around.
Since FIAT is new over here, they'll bring a lot of whatever has the best chance of selling. If all goes well, I think their range of engines could stand an increase. Either giving us smaller displacements, diesels, or even new upspecced engines made in chrysler plants just for our market. But that's all later on.
edit: I guess the one big variable is a question of price. If the smallest engined 500's can compete with the Fortwo's low $12k base msrp, they may be a very reasonable alternative in low volumes. I say the FIAT 500, since I haven't heard anything about non-4x4 pandas coming yet.