Marshalls wpm during his peak is one factor, his unmatched skill set is another. He was legitimately express, could swing it, was economical and was, when the mood struck him the enforcer, he could and would hit you. He excelled vs everyone and in all conditions, home and away. Don't think any one else combined those traits and that resume.
I have McGrath over Hadlee because one primarily plied his trade in more or less helpful conditions and 3 countries, while the other went from the live to dead era of pitches with no noticable drop off in quality or production.
Him and Marshall were also the primary reasons and the x factors that propelled their teams to the two greatest of all time.
So yeah, Hadlee, easily the 3rd member of that top tier, but also clearly 3rd. Also played a bit vs minnows which no doubt helped his numbers a bit.
Steyn averaged over 27 vs 3 countries with double digit sample sizes and was often noted not to have a plan B. Not saying he isn't elite, but not in the upper top tier for me.
Barnes, first off don't rate players from that era. Second, he played vs only two teams where he was basically Ambrose was Australia, and then demolished a minnow to supercharge his numbers.
Two massive holes in his record, he's definitely in my 7 man top tier (anywhere from 5th to 6th), but can't be in the top 3 discussion for the goat in my humble opinion.