Seeing as no one has taken the actual test yet... its hard to say. Although the practice test by the AAMC has a lot of biochem - Stuff you would really need to take a class to understand and remember fully (in my opinion). According to the What’s on the MCAT2015 Exam? document that the AAMC released, first semester biochem will make up 25% of Chem/Phys section and 25% Bio/Biochem section. Thats 14-15 biochem questions per section, and 13-15% of the entire exam.
That said, you certainly can get a good score/know your biochem from the books alone - but I think it would be a safer bet if you took the class. Besides, most colleges offer a biochem class w/o lab so it really isn't too bad. Not to mention biochem integrates new material with important concepts from general bio and chemistry. It's super useful to know and it helps you remember the old stuff as well.