The CBIR is a cutdown version of the normal IR studies, with a total of 80 hours' study, but given the tendency to ask questions from the full IR you may as well do that anyway, as Rudestuff says. Not only that, with the CBIR you must do an advanced aeroplane performance course if you want to fly anything bigger than a light twin. Again, do the full thing, you are then covered.
And remember you can do as much flying training as you want while you are studying for the exams - you just can't take the skill test until you've passed them. A lot of schools don't seem to grasp this.