The real nonsense about the rating is that it assumes, a bit like Sea Bird II's that training methods have not improved. They clearly have and in my opinion if Arkle were to race Kauto he would be beaten, probably quite comfortably because he simply wouldn;t have been as fit or as well fed and conditioned. He may have had more talent but it's nonsensical trying to compare them.
Whats more Arkle might not have been half the horse he supposidly was when trained under such modern techniques with rivals fully match fit from proper training establishments - and not from hunting and galloping in ploughed fields.