Congrats on a nice buck! Thx for showing the autopsy pics as well.
As far as the ethics of hunting subsonic, I really don't see a performance difference between your load for the 300BO and a 44Special loaded with a 240swc at 800-950fps. Sure, its quieter (and that's not a problem for me) but does anyone think the deer will know the difference? The only true difference is the person pulling the trigger. If they're judgment, knowledge and skills are lacking then there will be problems.