Yup, you are completely right. I'm inside right nnow and I can't imagine how they would fit the whole IMAX stuff here. A bit disappointed, but hey! That's OK I got to see it with a few days early - with Christopher Nolan being here too.
By the way, if any of you has questions about the movie / the premiere, feel free to ask them: I'll try my best to answer them tomorrow.
Apologies for the poor quality but here's a quick shot I took of the temporary room we were staggered to see in the Mezzanine in the Grand Rex used to house the 70mm projection equipment for the premiere.
According the Rex's facebook page Mr Nolan paid a visit to this monolith erected for Interstellar ~5 minutes after the film had started. I have to say the sound system at the Rex was pretty phenonmenal. My expectation was that in such a large venue the sound would be noticeably coming from the direction of screen but it was truly all encompassing allowing Zimmer's score to flood the auditorium.
It was definitely worth the day trip from London to see it early. Just need to wait 'til 05:30 Friday morning to see it at the BFI's IMAX now to really get the full majesty of the visuals.
[ I couldn't possibly improve on the excellent content already in the Member's Reviews threads. Those guys nailed it. ]