I've been to a few theme parks in the USA.
Firstly, Islands of Adventure is amazing and probably my favourite theme park that I've visted. The theming of the entire park is absolutely incredible and it does truely feel that you're in a different world. For the record I'd probably say that their best themed ride there is Dudley do rights ripsaw falls (a flume ride,) which is really, really impressive.
Without riding the smiler its difficult to compare it with the Hulk or Dueling Dragons in terms of theming. However, one point I will make is that with a lot of coasters at islands of adventure l, the theming stops as soon as the coaster moves. I'd you think about the Hulk, as soon as you're launched out of the gamma ray, there's not a great deal of theming, and the Dueling dragons has pretty much nothing the second you leave the castle an ascend the lift hills. Compare tht with the likes of Nemesis and the Smiler, which has theming throughout, and I think there is a decent argument to say the theming at Alton Towers is on a parr with it. It's definately not on as grander scale, but this is an 18m investment not a 50m one, but it doesn't necessarily have to be absolutely huge and in your face to be good. Sometimes subtly is the key and the Smiler has a really interesting plot, storyline. We'll know more when the thing eventually opens I guess.
On a side note I went to six flags magic mountain last month and they barely theme anything so the idea of the USA theming being impressive doesnt apply to everything.