A 9300m and a 9400m are both just chips, and they both perform similarly. When paired with a second, smaller GPU in hybrid SLI (or similar technology involving switching GPUs) you can improve battery life by only using the high-power GPU when you need it. The 9400 is not an enthusiast GPU, though, and it's usually paired with more powerful graphics cards (like the 9800) and used as the low power one. I don't know where you read that the macs are using dual GPUs, but they aren't (as far as I can tell, at least. It's not like they have a need for one...). Any 9300m will perform almost identically to any 9400m.
Where did you read that the 9400 was paired with a smaller GPU for the hybridization? The 9400/9300 series is nvidia's smallest GPU, I don't know what else they'd be paired with. (unless you can use an Intel chip with one, which I'm not aware of being possible)