This is debateable... It's one of those things that if you ask 10 people, you'll get 10 answers. Maybe with the routing in the MS3 it makes a difference, but in the Subaru world, a divided DP doesn't offer any real gains over a good bellmouth. And supposedly, there were some tests showng no real gains over the stock design which is just a plate and a single pipe. *shrug*
I have a divided on my WRX and will likely end up with a bellmouth on my STI. From what I've seen, I don't think there's any difference. But I definitely prefer them over the stock design.