BrM is the reference spec since it's the most relevant taking in consideration the survival.
- About Magmaw, since it's a Cataclysm boss, we do consider the BrM as stronger. The fact the gear is close enough from WW spec shouldn't annoy you. The ilvl written (460+ in the case of Magmaw), is, more or less, a limit to the encounter. I do practice the encounters with a reference ilvl 463 for making estimations about what min/max ilvl is required.
- About legendaries, since it's mainly raids before Cata, it's possible to solo them with a WW spec (faster and easier in most cases) : BT/Uldu... Yeah maybe Uldu 25 should be done with a BrM spec, before having a decent gear (550+ for WW it, 515+ for BrM), cuz there's quite a strong amount of HPs/damages/abilities... Yogg still requires a very strong ilvl in comparison with the rest (smthg like 550+ for Brm/ 560+ for WW). All really old stuff (MC/BT...) is really very doable regardless of the spec used.
- About the Celestials, it's only BrM spec, and it's at 550+/560+ ilvl. I should add though, would avoid misunderstanding
. Maybe MW at higher ilvl, but I still don't play enough my MW spec to tell with certitude
.