Hey Yan,
I run the Hatori SAB55 on my OS55 and it's a good combination, powerful and smooth. I'm hearing good reports on the new Powerboost2 50/55 pipe from OS recently so i will be giving one of these a try shortly.
I could'nt comment on the MP5SB as i've never run that particular pipe, but the newer range of pipes designed for the current breed of 55 class engines should allow you to extract the extra power that the 55 can deliver.
Cheers!
Ross