I've had an extensive MACTing career. Biochem definitely helps. Though honestly, the genetics on the MCAT is quite basic. Mostly limited to Mendelian genetics, pedigree analysis, and basic population stuff. You definitely need to know that stuff, but its not hard learning the genetics you need even if you haven't taken a class in it.
On the other hand, having background in biochem and especially cell biology is huge. Understanding the dissociation curves, knowing the amino acids or even just being familiar with them, knowing what second messengers are, the roles of Ca2+, etc. gives you a huge edge on the passages. The passages are often intended for you to deduce information with some basic background info but if you already know what they're talking about, then you're set. I agree Physiology is a must too.