Hi Margot, thanks for your question!!
Charles did a great job answering your question, but I'll add a few points.
iSci students end up knowing a lot. In terms of the calibre of the students: iSci students in the physics concentrations are the top students in physics at McMaster, those in chemistry, the top in chemistry, etc. We feel like we are in a bubble sometimes, but it is clear that iSci does a great job in preparing students for the content. Also, the key to iSci, and the whole reason why it is put together in one course is the "Integrating" aspect, which really and fundamentally is what is valuable to the program, and the type of learning that is built to be best for the students accepted to the program.
I took AP biology, physics, and calculus in high school. And yes, I found that a lot of the content in all three was repeated (like it is meant to be). The fun part for me, was that this allowed me to jump into the higher critical thinking, application, and analysis that is not covered with AP and IB programs a lot quicker, and I could more readily contribute to the groups. All this background will give you will less time reviewing content from the textbook and iCons, because the content is familiar.
Electives: totally depends on your elective. In my opinion, some are more challenging, some are less, but this all completely depends on your level of interest, learning style, and how well the elective suits you. This isn't high school anymore, it will be tough to do well in a subject/class you have no interest in, so really take those that you are willing to work for and learn about.