I was looking at M3/M4 myself because of S55.
Testdriven two of them:Pretty much perfect, but a tad too big for my likings.
But as you say each to their own, so when M2C with S55 came out I had to have it. I'm one of the only S55 freaks here, I think it sounds decent enough, revs so high and is so powerful and tunable, plus it has its cooling and stuff sorted out.
Though I'm on the brink of ordering a Akra axleback , Lightweight axleback or Remus axleback system. Probably Remus(I have good contacts with Dutch distributor and as we speak I got an email of them with some discount as well
)
Enjoy your M4, it's a blast.
Cheers
Robin