Citra, and citrus flavor in general, is definitely still very much in vogue for American IPAs, but I don't really think of there being a huge difference in carbonation levels. British beer is usually less carbonated than American as a rule, but carbonation is actually one of the few areas where often-extreme American IPAs are relatively restrained, so it's not this massive gulf.

"Imperial" is a tough descriptor for a British style, at least in America, because here it's usually used to signify something akin to "double": more alcoholic, more hoppy, just generally "more."

Well, I concede that it may be even more exaggerated in other American styles, but certainly American IPAs are very much more carbonated than IPAs in Britain that aren't imitating that style.

'Imperial IPA' in Britain denotes a more traditional style - of course it's not regulated, but the one thing you can be certain of is that it won't be in the 'American IPA' style (you can get British made IPAs sold as 'American IPA', i.e. it is meant as a classifier of style, not orgin).

Traditional IPAs are low in carbonation even among British ales, highly hopped (boiled and dry), and 6-9%.

In short, the way you're saying it's used in America doesn't sound wrong to me!

(I believe dry hopping actually originated with the IPA, and its long voyage from London docks to India.)