In essence, the powerfist wording says regardless of anything, you strike at initiative 1. Meanwhile the doom siren says regardless of Powerfist rules, you strike first.
It actually just says regardless of weapons, which would include the powerfist. So Sirens say to ignore powerfists, and powerfists say to ignore everything, which would have to include Sirens. Hence the contradiction. Neither can really be said to be more specific.