But that type of composition would have a difference spectrum.
(different?) Could be.
The known spectra of comets and planets are based on reflected light. None (that I know of) are from radiated light at 7,000 - 1,000,000 Kelvin.
Either way, a sun with iron, nickel, oxygen, and silicon would be closer in composition than would be the standard hydrogen - helium theory.
Again, assuming the sun, planets, comets, etc. were formed by accretion form the same ball of matter.