The digital "liemax" theaters use dual 2k projectors and have a 1.9:1 aspect ratio (real imax is 1.44:1). The biggest of the digital imax screens still aren't as big as the 15/70 screens, even if you're just talking about the width. What Imax did was to take the biggest auditorium at a multiplex and renovate it to make the screen go from wall to wall (which it was almost doing anyway) and floor to ceiling. They also removed the first several rows of seats to make the screen closer to the audience.
I'm not sure how the 2k+2k overlap works for 2d movies, but imax argues that it yields an image greater than 4k. In any event, you're still seeing the dmr'd version of the movie, which is supposed to make everything look better whether it's on actual 15/70 film or dual 2k dlp.
For movies that were shot in actual 65mm imax, like Ghost Protocol, I can only imagine that the effect is greatly diminished in the digital theaters. Smaller screen, lower resolution, cropped aspect ratio, and multiplex seating are all pretty far removed from the genuine article.
Star Trek Into Darkness was the first time a movie had been shot in imax and then converted into 3d. Paramount did this because we're in a post-Avatar movie business now. When Nolan wanted to shoot some of The Dark Knight in Imax back in 2007, that's pretty much all there was aside from 35mm and the equivalent digital projection. 3D hadn't truly "arrived," it was only in a few theaters here and there. Cut to 2012, now there are thousands of 3D theaters and only several hundred imax. Stuff like Ghost Protocol and Dark Knight Rises were now the exception to the rule. The moneymaking potential of 3D now outweighed that of imax.
I never got around to seeing Into Darkness in 15/70 out in Chantilly, although I heard the imax shots were matted to 1.66:1 for some reason. In an interview with the vfx guys they said this was to avoid making the AR switch too jarring for the audience. I can't help but think the real reason had something to do with rendering times and all that, but who knows.