I think that your observation about slaves being capital investments, and therefore not generally physically abused is probably pretty valid. It was more cost efficient to hire Irish for the more dangerous jobs. Afterall, the Irishman was working by free will and could be discharged when he got injured, or the job was finished. I'm not whining about the Irish in particular, I know lots of Americans of Irish extraction and none of them are planning to seek reparations.
I had to laugh when I heard that africanhyphenamericanhyphenfemale lawyer interviewed on the news. She spoke as though the results of their suit is a foregone conclusion.